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I 



UNDERSTANDING 

CLIMATIC 

CHANGE 



^■m 



NATIONAL ACADEMY OF SCIENCES 



UNITED STATES COMMITTEE FOR THE 
GLOBAL ATMOSPHERIC RESEARCH PROGRAM 

National Research Council 



UNDERSTANDING 

CLIMATIC 

CHANGE 

A Program for Action 



NATIONAL ACADEMY OF SCIENCES 

WASHINGTON, D.C. 
1975 



notice: The project which is the subject of this report was approved by the Gov- 
erning Board of the National Research Council, acting in behalf of the National 
Academy of Sciences. Such approval reflects the Board's judgment that the project 
is of national importance and appropriate with respect to both the purposes and 
resources of the National Research Council. 

The members of the committee selected to undertake this project and prepare 
this report were chosen for recognized scholarly competence and with due con- 
sideration for the balance of disciplines appropriate to the project. Responsibili v 
for the detailed aspects of this report rests with that committee. 

Each report issuing from a study committee of the National Research Council is 
reviewed by an independent group of qualified individuals according to procedures 
established and monitored by the Report Review Committee of the National 
Academy of Sciences. Distribution of the report is approved, by the President of 
the Academy, upon satisfactory completion of the review process. 



The activities of the United States Committee for the Global Atmospheric Re- 
search Program leading to this report have been supported by the National Oceanic 
and Atmospheric Administration and the National Science Foundation under Con- 
tract NSF-C310, Task Order No. 197. 



Library of Congress Cataloging in Publication Data 

United States Committee for the Global Atmospheric Research Program. 
Understanding climatic change. 

Includes bibliographies. 

1. Climatic changes — Research. I. Title. 
QC981.8.C5U54 1975 551.6 75-827 
ISBN 0-309-02323-8 



Available from 

Printing and Publishing Office, National Academy of Sciences 
2101 Constitution Avenue, Washington, D.C. 20418 



Printed in the United States of America 



U.S. COMMITTEE FOR THE GLOBAL ATMOSPHERIC 
RESEARCH PROGRAM 

Scientific Members 

Verner E. Suomi, University of Wisconsin, Chairman 

Richard J. Reed, University of Washington, V ice-Chairman 

Francis P. Bretherton, National Center for Atmospheric Research 

T. N. Krishnamurti, Florida State University 

Cecil E. Leith, National Center for Atmospheric Research 

Richard S. Lindzen, Harvard University 

Syukuro Manabe, Geophysical Fluid Dynamics Laboratory (noaa) 

Yale Mintz, University of California at Los Angeles 

Allan R. Robinson, Harvard University 

Joseph Smagorinsky, Geophysical Fluid Dynamics Laboratory (noaa) 

William L. Smith, National Environmental Satellite Service (noaa) 

Ferris Webster, Woods Hole Oceanographic Institution 

Michio Yanai, University of California 

John A. Young, University of Wisconsin 

John S. Perry, National Research Council, Executive Scientist 

John R. Sievers, National Research Council, Executive Secretary 

Ex-officio Members 

John W. Firor, National Center for Atmospheric Research 
Robert G. Fleagle, University of Washington 
Thomas F. Malone, Holcomb Research Institute 

Invited Participants 

Charles W. Mathews, National Aeronautics and Space Administration 
Gordon H. Smith, Department of Defense 
Edward P. Todd, National Science Foundation 

John W. Townsend, Jr., National Oceanic and Atmospheric Administra- 
tion 
Robert M. White, Department of Commerce 

Liaison Representatives 

Eugene W. Bierly, National Science Foundation 

William Chapin, Department of State 

Rudolf J. Engelmann, Atomic Energy Commission 

Albert Kaehn, Jr., Department of Defense 

Douglas H. Sargeant, National Oceanic and Atmospheric Administration 

Joseph F. Sowar, Federal Aviation Administration 

Morris Tepper, National Aeronautics and Space Administration 

iii 



PANEL ON CLIMATIC VARIATION 



Members 



W. Lawrence Gates, The Rand Corporation, Co-Chairman 

Yale Mintz, University of California at Los Angeles, Co-Chairman 

Wallace S. Broecker, Lamont-Doherty Geological Observatory 

Kirk Bryan, Geophysical Fluid Dynamics Laboratory 

Jule G. Charney, Massachusetts Institute of Technology 

George H. Denton, University of Maine 

Harold C. Fritts, University of Arizona 

John Imbrie, Brown University 

Robert Jastrow, Goddard Institute for Space Studies 

Edward N. Lorenz, Massachusetts Institute of Technology 

Syukuro Manabe, Geophysical Fluid Dynamics Laboratory 

J. Murray Mitchell, Jr., Environmental Data Service 

Jerome Namias, Scripps Institution of Oceanography 

Henry Stommel, Massachusetts Institute of Technology 

Warren M. Washington, National Center for Atmospheric Research 

Consultants 

John E. Kutzbach, University of Wisconsin 
Cecil E. Leith, National Center for Atmospheric Research 
Abraham H. Oort, Geophysical Fluid Dynamics Laboratory 
Richard C. J. Somerville, National Center for Atmospheric Research 



IV 



FOREWORD 






The Preface of the U.S. Committee for the Global Atmospheric Research 
Program document Plan for U.S. Participation in the Global Atmos- 
pheric Research Program begins : 

In late 1967 the International Council of Scientific Unions, acting jointly with 
the World Meteorological Organization, proposed a Global Atmosphere Research 
Program (garp) to accomplish the objectives stated in U.N. Resolutions 1721 
(XVI) and 1802 (XVII), namely, "to advance the state of atmospheric sciences 
and technology so as to provide greater knowledge of basic physical forces affect- 
ing climate . . . ; to develop existing weather forecasting capabilities . . . ," and 
"to develop an expanded program of atmospheric science research which will com- 
plement the program fostered by the World Meteorological Organization." 

Now, in 1974, a program to reach the "weather forecasting" objective of 
garp is well under way. In this report, the U.S. Committee for the 
Global Atmospheric Research Program (usogarp) outlines a program 
to "understand the basic physical forces affecting climate." There is 
ample evidence, summarized in Appendix A of this report, that climate 
does change, and there is more than ample evidence from past history 
and even recent events that changes in climate can profoundly affect 
human activities and even life itself. Indeed, as a growing population 
places ever greater demands on food and fiber resources, man's sensitiv- 
ity to variations in climate will increase. We have an urgent need for 
better information on global climate. Unfortunately, we do not have a 
good quantitative understanding of our climate machine and what deter- 
mines its course. Without this fundamental understanding, it does not 



VI UNDERSTANDING CLIMATIC CHANGE 

seem possible to predict climate — neither in its short-term variations nor 
in its larger long-term changes. There are some who believe that impor- 
tant variations in climate can occur with changes in the controlling fac- 
tors that are so small they are difficult to measure. With such barriers 
to be overcome, is there any assurance of success? We believe so. 

First, the two garp objectives, dealing with weather and climate, are 
strongly related to each other. A better understanding of the physical 
processes that affect one means a better understanding of the processes 
that affect the other. The difference lies mainly in how the processes 
should be taken into account. Mathematical models fashioned to take 
into account long-term changes will have to have some characteristics 
that are different from those fashioned mainly for short-term (weather) 
changes. There has been significant progress in garp's weather objec- 
tive; therefore, there already has been important progress in garp's 
climate objective. 

Second, there has been a tremendous improvement in our ability to 
observe the global weather, thanks to weather satellites. By the end of 
this decade, we will have the ability to observe the entire earth with 
needed meteorological observations. An ability to obtain better weather 
observations is an ability to obtain better climate observations also. 
Important as this is, meteorological satellites also allow us to monitor 
those parameters that we now believe control the climate machine: the 
sun's output, the earth's albedo, the distribution of clouds, the fields of 
ice and snow, and the temperatures of the upper layers of the ocean. 
These parameters control the average state of the weather and thus 
climate. Meteorological satellites are observing some of these param- 
eters and could measure all of them. In some instances, the data are 
already being collected. These now need to be assembled to serve the 
needs of climate research. The feasibility of collecting data from ocean 
platforms has been established. A program to do it is needed. This 
report outlines the key requirements. 

Third, research into past climates has made significant advances. We 
now not only know what happened in the past far better than we did a 
decade or two ago, but these data will provide an important information 
base against which theories and numerical models of climate can be 
tested. 

Last, but far from least, there is a new generation of atmospheric sci- 
entists. Their tools are the computer, numerical models, and satellites, 
and they know how to use them well. The usc-garp believes that this is 
an adequate manpower base. We do not expect any breakthroughs, and 
progress could be slower than desired, but the program outlined in this 



FOREWORD VII 

document is a rational approach toward obtaining progress as rapidly as 
possible on this vital subject. 

The usc-garp further believes that neither the scientific community 
nor the nation can afford to be complacent with its present level of 
understanding on this important aspect of the earth's physical environ- 
ment. The natural forces determining the world's weather and climate 
are beyond our control, but having better insight into what nature might 
do should help the nation to plan for what it must do. 

This report was prepared by the Committee's Panel on Climatic 
Variation. On behalf of the usc-garp, I express full appreciation to its 
Co-Chairmen, Yale Mintz and W. Lawrence Gates, and all of its mem- 
bers for this important report. 

verner e. suomi, Chairman 

U.S. Committee for the 

Global Atmospheric Research Program 



PREFACE 



The increasing realization that man's activities may be changing the 
climate, and mounting evidence that the earth's climates have undergone 
a long series of complex natural changes in the past, have brought new 
interest and concern to the problem of climatic variation. The impor- 
tance of the problem has also been underscored by new recognition of 
the continuing vulnerability of man's economic and social structure to 
climatic variations. Our response to these concerns is the proposal of a 
major new program of research designed to increase our understanding 
of climatic change and to lay the foundation for its prediction. 

The need for increased understanding of the physical basis of climate 
was recognized by the Panel on International Meteorological Coopera- 
tion of the Committee on Atmospheric Sciences in its report of 1966, 
which led to the development of the Global Atmospheric Research Pro- 
gram (garp). This objective was embodied in the garp plan as a "sec- 
ond objective" devoted to the study of the physical basis of climate, to 
be undertaken along with the program's primary concern of improving 
and extending weather forecasts with the aid of numerical models. 

In March 1972, the United States Committee for garp appointed 
the Panel on Climatic Variation to study the problem and to submit 
recommendations appropriate for climatic objectives of garp observa- 
tional programs, particularly the First garp Global Experiment (fgge) 
planned for 1978. The Panel's charge was subsequently enlarged to 
include recommendations for the design and implementation of a national 
climatic research program. 

ix 



X UNDERSTANDING CLIMATIC CHANGE 

In its initial deliberations, the work of the Panel seemed logically to 
fall into three categories, depending on the time scale of climatic varia- 
tion. First, the shorter-period variations, of the order 10 1 to 10 years, 
which are documented by modern instrumental observations; second, the 
variations of intermediate length, of the order 10 to 10 3 years, which are 
largely documented by historical and proxy data sources; and third, the 
longer-period variations, of the order 10 3 years and beyond, for which 
documentation comes from paleoclimatic and geological records. Three 
subpanels were therefore formed, and a report was issued in February 
1973 by the subpanel concerned with monthly to decadal time scales 
(W. L. Gates, Chairman), which is the basis of the main body of the 
present report. The deliberations of the subpanels concerned with 
decadal to millenial changes (J. M. Mitchell, Chairman) and with 
millenial changes and beyond (W. S. Broecker, Chairman) were the 
basis of Appendix A of this report. 

From the beginning of the Panel's work it was realized that it would 
be necessary to address a wide range of questions involving the use of 
climatic data from instrumental and proxy sources, the use of numerical 
simulation models, and the conduct of research on the physical mecha- 
nisms of climatic change. It was also obvious in undertaking an assign- 
ment of this magnitude that the Panel would not be able to refer to the 
large number of studies that have an important bearing on the problem 
of climatic variation. We have, therefore, generally cited only those 
works that were useful in framing our recommendations and in making a 
brief overview of present research (see Chapter 5). Some of our 
recommendations have been made previously by other groups [see, for 
example, C. L. Wilson (Chairman), 1971: Study of Man's Impact on 
Climate (smic) Report, Inadvertent Climate Modification, W. H. 
Matthews, W. W. Kellogg, and G. D. Robinson, eds. Massachusetts 
Institute of Technology, Cambridge, Mass.], and we are also aware that 
the problem of climatic change has been considered by several other 
groups and is of concern to other committees of garp. 

In addition to the contributions of the Panel's members, a number of 
consultants to the Panel also made valuable contributions: J. E. Kutz- 
bach and A. H. Oort on the observational and statistical aspects of 
climatic change; C. E. Leith on the question of climatic predictability; 
and R. C. J. Somerville on the evaluation of numerical model perfor- 
mance. A. R. Robinson of Harvard University also contributed material 
on the role of the oceans in climatic change. Useful comments on various 
aspects of the Panel's work were also made by S. H. Schneider of the 
National Center for Atmospheric Research; by E. W. Bierly, J. O. 
Fletcher, and U. Radok of the National Science Foundation; by R. S. 



PREFACE XI 

Lindzen of Harvard University; by R. J. Reed and R. G. Fleagle of the 
University of Washington; and by J. Smagorinsky of the Geophysical 
Fluid Dynamics Laboratory. The organization and preparation of the 
report as a whole was undertaken by W. L. Gates. 

Appendix A, which is a survey of past climatic variations, was pre- 
pared principally by J. Imbrie, W. S. Broecker, J. M. Mitchell, Jr., and 
J. E. Kutzbach. The portions of this Appendix concerned with den- 
drochronology were prepared by H. C. Fritts, and those concerned with 
glaciology by G. H. Denton. Unpublished data and figures used in this 
Appendix were also kindly supplied by A. H. Oort; C. Sancetta of Oregon 
State University; A. Mclntyre, J. D. Hays, and G. Kukla of the Lamont- 
Doherty Geological Observatory; V. C. LaMarche of the University of 
Arizona; J. Kennett of the University of Rhode Island; and T. Kellogg, 
N. G. Kipp, R. K. Matthews, and T. Webb of Brown University. 

Appendix B, which presents a comparative review of selected climate 
simulation capabilities of global general circulation models, was prepared 
principally by W. L. Gates, K. Bryan, and W. M. Washington. Valuable 
comments and contributions of unpublished material were also made by 
S. Manabe, R. C. J. Somerville, Y. Mintz, and R. C. Alexander of The 
Rand Corporation. 

This report makes no claim to completeness, and many important mat- 
ters are not touched upon. For example, we have not considered the 
questions of instrumental design and logistical support necessary to carry 
out the observational programs that we have recommended, nor have 
we dealt with the training and educational activities necessary to supply 
the additional scientific manpower. Although we have presented some 
thoughts on possible organizational arrangements for the conduct of the 
necessary research, and have made some preliminary cost estimates, such 
questions were regarded as being outside the scope of the Panel's im- 
mediate objectives and responsibility. 

The principal purpose of this report is to recommend a comprehen- 
sive research program, which we feel is necessary to increase significantly 
our understanding of climatic variation, and the Panel will consider its 
efforts to have been successful if the report serves as a useful planning 
document to this end. In making its recommendations, the Panel is 
aware of what has been called the problem of "(don't know)," 2 i.e., 
those who are called on to implement the program may not know that 
we don't know the answers to the central questions. The presentation of 
this report at least makes it clear that we don't know, and thereby re- 
duces the exponent to unity. The successful execution of the program 
should remove at least part of the remaining "don't know." In short, we 
have attempted to describe here what should be done, and recognize 



XII UNDERSTANDING CLIMATIC CHANGE 

that what can be done and then what actually will be done remain to 
be determined. 

We wish to acknowledge the valuable advice and assistance of John 
R. Sievers of the National Research Council and of Verner E. Suomi of 
the University of Wisconsin throughout the preparation of this report. 
We are also indebted to Viv Pickelsimer of The Rand Corporation for 
her efficient handling of many of the details of the Panel's work and the 
preparation of the typescript. 

W. LAWRENCE GATES 
YALE MINTZ 

Co-Chairmen, Panel on Climatic Variation 



CONTENTS 



INTRODUCTION 

Limits of Our Present Knowledge / 2 

Need for Data; Need for Understanding; Need for Assessment 
Future Efforts and Resources / 5 

Research Approaches; The Question of Priorities 
Purposes and Contents of This Report / 7 



2 SUMMARY OF PRINCIPAL CONCLUSIONS AND 
RECOMMENDATIONS 



PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 13 

Climatic System / 13 

Components of the System; Physical Processes of Climate; Defi- 
nitions 
Causes of Climatic Change / 20 

Climatic Boundary Conditions; Climatic Change Processes and 

Feedback Mechanisms; Climatic Noise 
Role of the Oceans in Climatic Change / 25 

Physical Processes in the Ocean; Modeling the Oceanic Circulation 
Simulation and Predictability of Climatic Variation / 28 

Climate Modeling Problem; Predictability and the Question of 

Transitivity; Long-Range or Climatic Forecasting 

xiii 



XIV UNDERSTANDING CLIMATIC CHANGE 

4 PAST CLIMATIC VARIATIONS AND THE PROJECTION 

OF FUTURE CLIMATES 35 

Importance of Studies of Past Climates / 35 

Record of Instrumentally Observed Climatic Changes / 36 

Historical and Paleoclimatic Record / 37 

Nature of the Evidence; Summary of Paleoclimatic History 
Inference of Future Climates from Past Behavior / 40 

Natural Climatic Variations; Man's Impact on Climate 



SCOPE OF PRESENT RESEARCH ON CLIMATIC 

VARIATION 46 

Climatic Data Collection and Analysis / 46 

Atmospheric Observations; Oceanic and Other Observations; Ob- 
servational Field Programs 

Studies of Climate from Historical Sources / 49 

Studies of Climate from Proxy Sources / 50 

General Syntheses; Chronology; Monitoring Techniques; Proxy 
Data Records and Their Climatic Inferences; Institutional Pro- 
grams 

Physical Mechanisms of Climatic Change / 54 

Physical Theories and Feedback Mechanisms; Diagnostic and Em- 
pirical Studies; Predictability and Related Theoretical Studies 

Numerical Modeling of Climate and Climatic Variation / 56 
Atmospheric General Circulation Models and Related Studies; 
Statistical-Dynamical Models and Parameterization Studies; 
Oceanic General Circulation Models; Coupled General Circulation 
Models 

Applications of Climate Models / 59 

Simulation of Past Climates; Climate Change Experiments and 
Sensitivity Studies; Studies of the Mutual Impacts of Climate and 
Man 



A NATIONAL CLIMATIC RESEARCH PROGRAM 62 

The Approach / 63 

What Climatic Events and Processes Can We Now Identify?; Why 

Is a Program Necessary? 
The Research Program (ncrp) / 66 

Data Needed for Climatic Research; Research Needed on Climatic 

Variation; Needed Applications of Climatic Studies 
The Plan / 94 

Subprogram Identification; Facilities and Support; Timetable and 

Priorities within the Program; Administration and Coordination 



CONTENTS XV 

A Coordinated International Climatic Research Program 
(icrp) / 105 
Program Motivation and Structure; Program Elements; Program 
Support 



REFERENCES 111 

APPENDIX A: SURVEY OF PAST CLIMATES 127 

Introduction / 127 

Nature of Paleoclimatic Evidence; Instrumental and Historical 
Methods of Climate Reconstruction; Biological and Geological 
Methods of Climate Reconstruction; Regularities in Climatic Series 

Chronology of Global Climate / 148 

Period of Instrumental Observations; The Last 1000 Years; The 
Last 5000 Years; The Last 25,000 Years; The Last 150,000 Years; 
The Last 1,000,000 Years; The Last 100,000,000 Years; The Last 
1,000,000,000 Years 

Geographic Patterns of Climatic Change / 1 63 

Structure Revealed by Observational Data; Structure Revealed by 
Paleoclimatography 

Summary of the Climatic Record / 1 79 

Future Climate : Some Inferences from Past Behavior / 1 82 
Potential Contribution of Sinusoidal Fluctuations of Various Time 
Scales to the Rate of Change of Present-Day Climate; Likelihood 
of a Major Deterioration of Global Climate in the Years Ahead 

References 190 

APPENDIX B: SURVEY OF THE CLIMATE SIMULATION 
CAPABILITY OF GLOBAL CIRCULATION MODELS 196 

Introduction / 196 

Development and Uses of Numerical Modeling / 198 

Atmospheric General Circulation Models / 201 

Formulation; Solution Methods; Selected Climatic Simulations 
Oceanic and Coupled Atmosphere-Ocean General Circulation 
Models / 218 

Formulation; Solution Methods; Selected Climatic Simulations; 

Coupled Ocean-Atmosphere Models 

References 236 






1 



INTRODUCTION 



Climatic change has been a subject of intellectual interest for many 
years. However, there are now more compelling reasons for its study: 
the growing awareness that our economic and social stability is pro- 
foundly influenced by climate and that man's activities themselves may 
be capable of influencing the climate in possibly undesirable ways. The 
climates of the earth have always been changing, and they will doubtless 
continue to do so in the future. How large these future changes will be, 
and where and how rapidly they will occur, we do not know. 

A major climatic change would force economic and social adjustments 
on a worldwide scale, because the global patterns of food production 
and population that have evolved are implicitly dependent on the climate 
of the present century. It is not primarily the advance of a major ice 
sheet over our farms and cities that we must fear, devastating as this 
would be, for such changes take thousands of years to evolve. Rather, 
it is persistent changes of the temperature and rainfall in areas com- 
mitted to agricultural use, changes in the frost content of Canadian and 
Siberian soils, and changes of ocean temperature in areas of high nutri- 
ent production, for example, that are of more immediate concern. We 
know from experience that the world's food production is highly de- 
pendent on the occurrence of favorable weather conditions in the 
"breadbasket" areas during the growing seasons. Because world grain 
reserves are but a few percent of annual consumption, an unfavorable 
crop year, such as occurred in the Ukraine in 1972, has immediate inter- 
national consequences. The current drought in parts of Asia and in 



£ UNDERSTANDING CLIMATIC CHANGE 

central Africa is producing severe hardship and has already caused the 
migration of millions of people. 

As the world's population grows and as the economic development 
of newer nations rises, the demand for food, water, and energy will 
steadily increase, while our ability to meet these needs will remain sub- 
ject to the vagaries of climate. Most of the world's land suitable for 
agriculture or grazing has already been put to use, and many of the 
world's fisheries are being exploited at rates near those of natural re- 
plenishment. As we approach full utilization of the water, land, and air, 
which supply our food and receive our wastes, we are becoming in- 
creasingly dependent on the stability of the present seemingly "normal" 
climate. Our vulnerability to climatic change is seen to be all the more 
serious when we recognize that our present climate is in fact highly 
abnormal, and that we may already be producing climatic changes as a 
result of our own activities. This dependence of the nation's welfare, as 
well as that of the international community as a whole, should serve 
as a warning signal that we simply cannot afford to be unprepared for 
either a natural or man-made climatic catastrophe. 

Reducing this climatic dependency will require coordinated man- 
agement of the nation's resources on the one hand and a thorough 
knowledge of the climate's behavior on the other. It is therefore essential 
that we acquire a far greater understanding of climate and climatic 
change than we now possess. This knowledge will permit a rational 
response to climatic variations, including the systematic assessment 
beforehand of man-made influences upon the climate and will make pos- 
sible an orderly economic and social adjustment to changes in climate. 

LIMITS OF OUR PRESENT KNOWLEDGE 

Although we have considerable knowledge of the broad characteristics 
of climate, we have relatively little knowledge of the major processes 
of climatic change. To acquire this knowledge it will be necessary to 
use all the research tools at our disposal. We must also study each com- 
ponent of the climatic system, which includes not only the atmosphere 
but the world's oceans, the ice masses, and the exposed land surface 
itself. Only in this way can we expect to make significant advances in 
our understanding of the elusive and complex processes of climatic 
change. 

Need for Data 

Observations are essential to the development of an understanding of 
climatic change; without them, our theories will remain theories and 



INTRODUCTION O 

the potential uses of our models will remain untapped. Our observa- 
tional records must be extended in both space and time, so that we 
can adequately document the climatic events that have occurred in the 
past, and so that we can monitor the climatically important physical 
processes that are now going on around us. Much of the present climatic 
data are of limited availability and need to put into forms that permit 
the systematic determination of appropriate climatic statistics and the 
assessment of the practical consequences of climatic variation. It is 
especially important that climatic data be organized and assembled 
to permit their use in conjunction with dynamical climate models. 

The oceans in particular exert a powerful influence on the earth's 
climates, yet we have inadequate oceanographic observations on the 
space and time scales needed for climatic studies. The important heat, 
moisture, and momentum exchanges that occur at the sea surface, and 
the corresponding transports that occur within the ocean, are not at 
all well known. Recent observations from the Mid-ocean Dynamics Ex- 
periment (mode) reveal energetic oceanic mesoscale motions at sub- 
surface levels, and our ignorance becomes even greater than we thought 
it was. 

The present international network of conventional meteorological 
observations has grown largely in response to the need for weather fore- 
casts, while most oceanographic data have been collected from ships 
widely separated in space and time. For the proposed research program, 
these data must be supplemented by truly global observations of the 
large-scale geophysical boundary conditions and of the physical pro- 
cesses that are important in climatic change. It is here that satellite 
observations are expected to play a key role, as they offer an unparalleled 
opportunity to monitor a growing list of variables, such as cloudiness, 
temperature, and the extent of ice and snow. Other climatically im- 
portant variables will require special monitoring programs, on either 
a global or regional basis. It is essential, moreover, that the relevant 
data be collected on a long-term basis in order to acquire the necessary 
statistics of climate. 



Need for Understanding 

Our knowledge of the mechanisms of climatic change is at least as 
fragmentary as our data. Not only are the basic scientific questions 
largely unanswered, but in many cases we do not yet know enough to 
pose the key questions. What are the most important causes of climatic 
variation, and which are the most important or most sensitive of the 
many processes involved in the interaction of the air, sea, ice, and land 
components of the climatic system? Although there is evidence of a 



4 UNDERSTANDING CLIMATIC CHANGE 

strong coupling between the atmosphere and the ocean, for example, we 
cannot yet say that we understand much about its consequences for 
climatic change. There are also indications in paleoclimatic data that 
the earth's climates may be significantly influenced by the long-term 
astronomical variations of the sun's radiation received at the top of 
the atmosphere. But here again we do not yet understand the processes 
that may be involved. 

There is no doubt that the earth's climates have changed greatly in 
the past and will likely change in the future. But will we be able to 
recognize the first phases of a truly significant climatic change when 
it does occur? Like the familiar events of daily weather, from which the 
climate is derived, climatic changes occur on a variety of space scales. 
These range from the change of local climate resulting from the removal 
of a forest, for example, to regional or global anomalies resulting from 
shifts of the pattern of the large-scale circulation. But unlike the 
weather, variations of climate take place relatively slowly, and we may 
think in terms of yearly, decadal, and millenial climatic changes. But 
the system is complex, and the search for order in the climatic record 
has only begun. 

Even the barest outline of a theory of climate must address the key 
question of the predictability of climatic change. This question is 
closely tied to the limited predictability of the weather itself and to 
the predictability of the various external boundary conditions and inter- 
nal transfer processes that characterize the climatic system. Although 
there is evidence of regularity on some time scales, the climatic record 
includes many seemingly irregular variations of large amplitude. How 
do we separate the genuine climatic signal from what may be un- 
predictable "noise," and to what extent are the noise and signal coupled? 
These are important questions, and ones to which there are no ready 
answers. The determination of the climate's predictability will require the 
further development and application of both theory and dynamical 
models, along with a greatly expanded data base. The answers, when 
they are found, will determine the limit to which we can hope to predict 
future climatic variations. 

Special attention must be paid to the fundamental role of the world's 
oceans in controlling the climate. The oceans not only are the primary 
source of the water in the atmosphere and on the land, but they consti- 
tute a vast reservoir of thermal energy. The timing and location of the 
exchange of this energy with the overlying air has a profound effect on 
the more rapidly varying atmospheric circulation. When the dynamics 
of this ocean-atmosphere interaction are better known, we may find 
that the ocean plays a more important role than the atmosphere in 
climatic changes. 



INTRODUCTION 



Need for Assessment 



We should add to these limits of our present knowledge the lack of 
comprehensive assessment of the impacts of climatic variation on human 
affairs. No one doubts that there are such impacts, for the specter of 
drought and the consequences of persistently severe winter weather are 
all too familiar in many parts of the world. Even so, we must admit that 
we cannot now adequately answer the question: What is a change of 
climate worth? A farmer may know what knowledge of the climatic 
conditions of the next growing season would be worth to him, but the 
answer in terms of national and international resource planning is more 
elusive. This lack of assessment is brought into sharper focus when we 
attempt to discern the economic and social consequences of possible 
alternative future climates. 

FUTURE EFFORTS AND RESOURCES 

Research Approaches 

Our future efforts must be guided by the realization that climatic 
changes in any one part of the world are manifestations of changes in 
the global climatic system. Since our fundamental goal is to increase 
our understanding of climatic variations to the point where we may 
predict (and possibly even control) them, we must subject our ideas 
to quantitative test wherever possible. 

The recent development of satellite-based observing systems, the 
coming of a new generation of high-speed computers, and the emergence 
of models suitable for climatic simulation combine to make such an 
undertaking feasible at this time. The importance of climatic variations 
requires, moreover, that we use all methods of inquiry that are likely to 
yield useful information, and that we do so at the earliest possible time. 

The principal approaches to the problem that are available to us 
are shown in Figure 1.1, and we recognize the importance of maintaining 
a balance of effort among them. These same approaches form the ele- 
ments of the climatic research program recommended in this report and 
broadly cover what we believe to be the needed efforts for observation, 
analysis, modeling, and theory. The successful execution of the program 
will require contributions from the physical sciences of meteorology, 
oceanography, glaciology, hydrology, astronomy, geology, and paleontol- 
ogy and from the biological and social sciences of ecology, geography, 
archeology, history, economics, and sociology. A program of this sort 
calls for a long-term commitment from the scientific research com- 
munity, from the sponsoring government agencies, and from the public. 



UNDERSTANDING CLIMATIC CHANGE 





Monitoring 






What is now . 






\ going on? / 




Numerical \ 
Models \ 




/ Empirical 
/ Studies 


What is shown 




How does the 


by climatic . 
simulations? / 


/Climatic \ 
/ Data Analysis\ 

^ What has V 


V system work? 




\ happened in / 


/ Future 


Theoretical \ 


\the past? / 


/ Climates 


Studies \ 




How and when 


How much do 




v is the climate 


we really / 




\ going to 


understand? / 


/ Climatic \ 

/ Impacts \ 

What does it 

all mean to 

man? 


\ change? 



FIGURE 1.1 The interdependence of the major components of a 
climatic research program and a number of key questions. 



The Question of Priorities 

The various components of the recommended climatic research program 
(fully described in Chapter 6) are to a great extent interdependent: 
data are needed to check the coupled general circulation models and to 
calibrate the simpler models; the models are needed to test hypotheses 
and to project future climates; monitoring is needed to check the pro- 
jections; and all are needed to assess the consequences. The question of 
priorities then becomes a matter of the priority of questions (see 
Figure 1.1), and there appear to be no a priori easy guidelines to relative 
importance. 

Our priorities are reflected in those actions and activities that we 
recommend be implemented at once and in those subsequent activities 



INTRODUCTION / 

for which planning should begin as soon as possible. While anticipating 
that much further planning will be necessary to implement the complete 
program, we urge that the essential interdependence of the various 
efforts be recognized and that all aspects of the problem be given support 
as parts of a coherent research program. 

PURPOSES AND CONTENTS OF THIS REPORT 

Broadly speaking, the purposes of this report are twofold: first, to 
advise the United States Government through the National Research 
Council's United States Committee for garp on the urgent need for a 
coherent national research program on the problem of climatic variation; 
and, second, to advise on the steps necessary to address the same prob- 
lem in the international scene. 

As noted previously, our response to the Government is the recom- 
mendation of a broadly based National Climatic Research Program 
(ncrp), whose goal is the resolution of the problem of climatic varia- 
tion. This program is presented in detail in Chapter 6, and its adoption 
is the first of our major national recommendations summarized in 
Chapter 2. In view of the possibly great impacts of future climatic varia- 
tions on the nation's welfare, we believe that it is our responsibility 
to call for a national commitment to this effort. We accordingly urge 
strongly that resources to carry out such a program be made available 
at the earliest possible time, including provision for the necessary ob- 
servations, computers, and research facilities. 

Our further response to the appropriate international bodies is the 
proposal of a coordinated International Climatic Research Program 
(icrp), which we believe to be a suitable mechanism for the pursuit 
of the climatic aspects of garp. As discussed in Chapter 6, we view this 
as a new program of considerably greater breadth than the present garp 
activities, but one for which the garp is a necessary prelude. The U.S. 
national program (ncrp) would form an integral part of the icrp, as 
would the national programs of other countries. In addition, we recom- 
mend a number of supporting programs whose observational require- 
ments may impact on the First garp Global Experiment scheduled for 
1978-1979. 

The remainder of this report consists of ( 1 ) a summary of our princi- 
pal conclusions and recommendations (Chapter 2); (2) a discussion 
of the physical basis of climate and climatic change (Chapter 3); (3) a 
summary of past climatic variations as drawn from the instrumental and 
paleoclimatic record (Chapter 4); (4) a brief review of the scope of 
present research on climatic variation (Chapter 5); and (5) the pro- 



8 UNDERSTANDING CLIMATIC CHANGE 

posed climatic research program (Chapter 6). Two technical appendixes 
prepared specially for this report present further details of the record 
and interpretation of past climates (Appendix A) and a brief com- 
parative review of the ability of present atmospheric and oceanic gen- 
eral circulation models to simulate selected climatic variables (Ap- 
pendix B). 



2 



SUMMARY OF PRINCIPAL CONCLUSIONS 
AND RECOMMENDATIONS 



The principal conclusions and recommendations that have resulted 
from the deliberations of this Panel, which are expanded upon else- 
where in this report, may be summarized as follows: 

1. To meet present and future national needs and to further the 
national contribution to garp, we strongly recommend the immediate 
adoption and development of a coherent National Climatic Research 
Program (ncrp) with appropriate international coordination. The 
major subprograms of the ncrp are summarized in Recommendations 
2, 3, and 4. 

2. To perform the needed analysis of selected climatic data, includ- 
ing that from conventional instruments and satellites, historical records, 
and paleoclimatic data sources, we recommend the establishment of 
a Climatic Data Analysis Program (cdap) as a subprogram of the ncrp. 
This program's functions would be to facilitate and coordinate the 
preparation and maintenance of a comprehensive climatic data inven- 
tory, the development of selected climatic data banks, and the prepara- 
tion of suitable data analyses, based on both current and paleoclimatic 
data. 

To carry out these functions we recommend the development of new 
climatic data-analysis facilities with access to suitable computing and 
data processing and display equipment, as components of a national 
network for climatic data analysis. We envisage these facilities as work- 
ing closely with the various specialized climatic data depositories and 



10 UNDERSTANDING CLIMATIC CHANGE 

as an essential mechanism for the successful execution of the cdap and 
of related components of the overall national program. 

In response to immediate practical needs, we recommend the initia- 
tion and continued support of empirical and statistical studies of the 
impacts of climatic change on man's food, water, and energy supplies. 
Support should also be given to studies of the broader social and eco- 
nomic consequences of climatic variations. 

3. To acquire the needed data on the important boundary conditions 
and physical processes of climate, we recommend the development of a 
global Climatic Index Monitoring Program (cimp) as a second subpro- 
gram of the ncrp. This program's functions would include the monitor- 
ing and collection, on appropriate climatic time and space scales, of data 
on the components of the global heat balance (including the solar 
constant), the ocean-surface temperature and the thermal structure of 
the surface mixed layer, the extent of ice and snow cover and other land- 
surface characteristics, the atmospheric composition and turbidity, 
anthropogenic processes, and, if possible, ocean-current transports and 
components of the hydrological cycle. This program will require a num- 
ber of new observational schemes in the atmosphere, in the ocean, and 
on land and will rely heavily on environmental satellites. We anticipate 
that such data will also have important uses on a real-time basis and 
that the cimp could serve as a national watchdog for climatic change. 

4. To accelerate research on climatic variation, and to support the 
needed development of climatic modeling on a broad front, we recom- 
mend the establishment of a Climatic Modeling and Applications Pro- 
gram (cmap) as a third subprogram of the ncrp. In this program, 
emphasis should be given to the development of coupled global climate 
models (cgcm's) of the combined atmospheric and oceanic general 
circulation and to the improvement of the models' treatment of clouds, 
mesoscale processes, and boundary-layer phenomena. Attention should 
also be given to the processes of air-sea interaction and to treatment 
of the ocean's surface layer, sea ice, and the oceanic mesoscale phe- 
nomena. We note the importance of extended model integrations to 
determine the annual and interannual variability of simulated climates 
and urge that appropriate studies be made of the sensitivity of simulated 
climates to physical and numerical uncertainties in the models' 
formulation. 

To provide the basis for the needed further modeling of climatic 
variation, we recommend the development and support of a wide variety 
of statistical-dynamical and other parameterized climate models. We 
note the importance of calibration in such models and urge that ap- 
propriate schemes be developed to permit extended climatic simula- 
tions which include oceanic and cryospheric variables. 



SUMMARY OF PRINCIPAL CONCLUSIONS AND RECOMMENDATIONS 11 

To provide the needed further insight into the mechanisms of climatic 
variation, we recommend the application of climatic models in support 
of empirical and diagnostic studies, with particular attention to the roles 
of climatic feedback processes in the coupled ocean-atmosphere sys- 
tem, to the questions of climatic predictability and transitivity, and to 
the climatic effects of changes in the geophysical boundary conditions. 

To provide the needed reconstruction of past climates and to develop 
a broader calibration of climate models, we recommend the initiation 
and support of systematic efforts to reconstruct selected events and 
periods in the climatic history of the earth. This should include the ap- 
plication of the cgcm's to simulate selected equilibrium paleoclimates 
and the use of statistical-dynamical or other parameterized climate 
models to infer the time-dependent evolution of the coupled atmosphere- 
hydrosphere-cryosphere climatic system. 

To further the needed application of climatic models, we recommend 
the systematic exploration with suitable climate models of a variety of 
possible future climates, due either to natural or man-made causes. 
These should include determination of the likely effects of changes in 
solar radiation, land-surface character, cloudiness, pollution, and ice 
extent. We urge that efforts be made to extract consistent physical 
hypotheses from such experiments and that the necessary statistical 
controls be developed. 

To lay the basis for the needed assessment of the possibilities of 
long-range or climatic forecasting, we further recommend the applica- 
tion of climate models of all types in experimental integrations using 
observed initial and boundary conditions. Appropriate climatic statistics 
should be drawn from such integrations and compared with observation 
insofar as possible, in order to establish the models' usefulness as long- 
range forecast tools. Initial emphasis should be given to the time periods 
of seasons to decades, for which there is presently the greatest practical 
need for scientifically based information. 

To assist in the performance of the needed research or climatic 
modeling and applications, we recommend that efforts be made to 
identify or form a number of cooperative research associations or cli- 
matic research consortia, which we view as natural and useful co- 
ordinating mechanisms for the effective performance and long-range 
stability of the ncrp. We further recommend that the period prior to 
1980 be used to develop additional scientific and technical manpower 
through the establishment and support of fellowships in appropriate 
areas of climatic research. 

5. In order to further the aims of the international garp efforts 
directed to the problem of climate and climatic variation, we recommend 
the adoption and development of an International Climatic Research 



12 UNDERSTANDING CLIMATIC CHANGE 

Program (icrp). By the very nature of climate, the U.S. national pro- 
gram is considered an integral part of the icrp, along with the climatic 
research programs of other nations. In view of the differences of the 
observational time scales and of the variables involved in weather fore- 
casting and climatic studies, and in view of the latter's broadly inter- 
disciplinary character, we visualize such a program being the logical 
successor to garp in matters relating to climate. Recognizing that the 
elements of the ncrp recommended above could equally well apply 
to an international program, we suggest that they be considered by 
the appropriate international organizations. 

To help provide the observational framework needed for climatic 
research, we recommend the designation of the period 1980-2000 as 
International Climatic Decades (icd). During this period, efforts should 
be made to secure broad international cooperation in the collection, 
analysis, and exchange of all available climatic data, including con- 
ventional observations and special data sets of particular climatic in- 
terest (such as during droughts and following volcanic eruptions). Dur- 
ing the icd we also recommend the initiation and support of regional 
climatic studies in order to describe and model local climatic anomalies 
of special interest. 

We further recommend development of appropriate national and in- 
ternational training programs and educational activities in order to 
promote the participation of all nations in climatic research. 

6. To provide the global paleoclimatic data needed for the recon- 
struction of past climates, we recommend the development of an Inter- 
national Paleoclimatic Data Network (ipdn) as a subprogram of the 
icrp. This program should aim to assist each nation in the cooperative 
identification, extraction, analysis, monitoring, and exchange of its 
unique paleoclimatic records, such as those from tree rings, soil types, 
fossil pollen, and data on sea and lake levels. 



3 



PHYSICAL BASIS OF CLIMATE 
AND CLIMATIC CHANGE 



CLIMATIC SYSTEM 

The term climate usually brings to mind an average regime of weather. 
The climatic system, however, consists of those properties and processes 
that are responsible for the climate and its variations and are illustrated 
in Figure 3.1. The properties of the climatic system may be broadly 
classified as thermal properties, which include the temperature of the 
air, water, ice, and land; kinetic properties, which include the wind and 
ocean currents, together with the associated vertical motions, and the 
motion of ice masses; aqueous properties, which include the air's 
moisture or humidity, the cloudiness and cloud water content, ground- 
water, lake levels, and the water content of snow and of land and sea 
ice; and static properties, which include the pressure and density of the 
atmosphere and ocean, the composition of the (dry) air, the oceanic 
salinity, and the geometric boundaries and physical constants of the 
system. These variables are interconnected by the various physical 
processes occurring in the system, such as precipitation and evaporation, 
radiation, and the transfer of heat and momentum by advection, con- 
vection, and turbulence. 



Components of the System 

In general terms the complete climatic system consists of five physical 
components — the atmosphere, hydrosphere, cryosphere, lithosphere, 
and biosphere, as follows : 

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PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 15 

The atmosphere, which comprises the earth's gaseous envelope, is 
the most variable part of the system and has a characteristic response 
or thermal adjustment time of the order of a month. By this we mean 
that the atmosphere, by transferring heat vertically and horizontally, 
will adjust itself to an imposed temperature change in about a month's 
time. This is also approximately the time it would take for the atmo- 
sphere's kinetic energy to be dissipated by friction, if there were no 
processes acting to replenish this energy. 

The hydrosphere, which comprises the liquid water distributed 
over the surface of the earth, includes the oceans, lakes, rivers, and 
the water beneath the earth's surface, such as groundwater and sub- 
terranean water. Of these, the world's oceans are the most important for 
climatic variations. The ocean absorbs most of the solar radiation that 
reaches the earth's surface, and the oceanic temperature structure repre- 
sents an enormous reservoir of energy due to the relatively large mass 
and specific heat of the ocean's water. The upper layers of the ocean 
interact with the overlying atmosphere on time scales of months to years, 
while the deeper ocean waters have thermal adjustment times of the 
order of centuries. 

The cryosphere, w '^h comprises the world's ice masses and snow 
deposits, includes the continental ice sheets, mountain glaciers, sea 
ice, surface snow cover, and lake and river ice. The changes of snow 
cover on the land are mainly seasonal and are closely tied to the 
atmospheric circulation. The glaciers and ice sheets (which represent 
the bulk of the world's freshwater storage) respond much more slowly. 
Because of their great mass, these systems develop a dynamics of their 
own, and they show significant changes in volume and extent over 
periods ranging from hundreds to millions of years. Such variations are, 
of course, closely related to the global hydrologic balance and to varia- 
tions of sea level (see Appendix A) . 

The lithosphere, which consists of the land masses over the surface 
of the earth, includes the mountains and ocean basins, together with 
the surface rock, sediments, and soil. These features change over the 
longest time scales of all the components of the climatic system, ranging 
up to the age of the earth itself. The processes of continental drift 
and sea-floor spreading, which have resulted in mountain building and 
in changes in the shapes and depths of the oceans, occur over tens and 
hundreds of millions of years. These events are not generally regarded 
as representing the same kind of interaction with other components of 
the system as the variations described above. We note, however, that 
there may be a significant relationship between the occurrence of major 
glacial periods and the times when continental land masses occupied 



16 UNDERSTANDING CLIMATIC CHANGE 

positions near the rotational poles of the earth (see Appendix A). 
The processes of isostatic adjustment and the accumulation of deep- 
ocean sediments also represent significant changes of the lithosphere, 
and as such may be viewed as earth-ice-ocean interactions. The intro- 
duction of volcanic debris into the atmosphere and its subsequent dis- 
persal may also be cited as an example of earth-air interaction. 

The biosphere includes the plant cover on land and in the ocean and 
the animals of the air, sea, and land, including man himself. Although 
their response characteristics differ widely, these biological elements 
are sensitive to climate and, in turn, may influence climatic changes. 
It is from the biosphere that we obtain most of the data on paleoclimates 
(see Appendix A). Natural changes in surface vegetation occur over 
periods ranging from decades to thousands of years in response to 
changes in temperature and precipitation and, in turn, alter the surface 
albedo and roughness, evaporation, and ground hydrology. Changes in 
animal populations also reflect climatic variations through the avail- 
ability of suitable food and habitat. The anthropogenic changes due 
to agriculture and animal husbandry are not known but may well be 
appreciable in altering at least regional climates. 



Physical Processes of Climate 

The climate at any particular time represents in some sense the average 
of the various elements of weather, along with the state of the other 
components of the system. The physical processes responsible for 
climate (as distinct from climatic change) are therefore basically the 
same as those responsible for weather. These processes are expressed 
in quantitative fashion by the dynamical equation of motion, the thermo- 
dynamic energy equation, and the equations of mass and water substance 
continuity, as applied to the atmosphere and ocean (see Appendix B). 
A process of primary importance for the circulation of the atmosphere 
and ocean is the rate at which heat is added to the system, the ultimate 
source of which is the sun's radiation. The atmosphere and ocean re- 
spond to this heating by developing winds and currents, which serve to 
transport heat from regions where it is received in abundance, such as 
in the equatorial and tropical areas, to regions where relatively little 
radiation is received, such as the polar regions of the earth. In this 
way, the atmosphere and ocean maintain the overall global balance 
of heat. A great deal of this heat is transported by the disturbances re- 
sponsible for much of our weather in middle and high latitudes, and 
similar disturbances may occur in the ocean. These eddies of the general 
circulation also participate in the transports necessary to maintain the 



PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 17 

global balances of momentum, mass, and the total quantity of water 
substance. 

While this simple view is a fair summary of our basic understanding 
of the general circulation, it is not without shortcomings. For example, 
it does not consider the basically different circulation regime in the 
low latitudes or the role of convective phenomena, and it does not 
consider the important variations of the circulation with height. It 
might also be noted that for other combinations of the planetary size 
and rotation rate, atmospheric composition, and meridional heating 
gradient, such as occur on other planets, an altogether different circula- 
tion regime — and hence climate — could result. 

Although the equations referred to above are fundamental in that 
they form the basis of our ability to simulate numerically the climate 
with dynamical models, they are not in themselves particularly reveal- 
ing as far as the more subtle physical processes of climate are con- 
cerned, to say nothing of the processes of climatic change. The heating 
rate is itself highly dependent on the distribution of the temperature and 
moisture in the atmosphere and owes much to the release of the latent 
heat of condensation during the formation of clouds and to the subse- 
quent influence of the clouds on the solar and terrestrial radiation. 
These processes, together with others that contribute to the overall heat 
balance of the atmosphere, are shown in Figure 3.2, in which data 
derived from recent satellite observations have been incorporated (see, 
for example, Vonder Haar and Suomi, 1971). Here the presence of 
clouds, water vapor, and C0 2 is seen to account for over 90 percent of 
the long-wave radiation leaving the earth-ocean-atmosphere system. 
This effective blocking of the radiation emitted by the earth's surface, 
commonly referred to as the greenhouse effect, permits a somewhat 
higher surface temperature than would otherwise be the case. It is 
interesting that this important effect is achieved by gases in the at- 
mosphere that exist in near trace amounts. 

We see from Figure 3.2 that the role played by clouds is an important 
one: the reflection and emission from clouds accounts for about 46 per- 
cent of the total radiation leaving the atmosphere; and in terms of the 
shortwave radiation alone, clouds account for two thirds of the 
planetary albedo. The largest single heat source for the atmosphere is 
that supplied by the release of the latent heat of condensation, and this 
is particularly important in the lower latitudes. There is also an ap- 
preciable supply of sensible heat from the oceans, especially in the 
middle and higher latitudes. It is therefore clear that water substance, in 
either vapor or droplet form, plays a dominant role in the atmospheric 
heat balance. And when we recall that the oceans themselves absorb 
















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PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 19 

most of the solar radiation reaching the surface, and that the presence 
of ice and snow also affect the heat balance, the climatic dominance 
of global water substance becomes overwhelming, even if ice is not 
taken into account. 



Definitions 

It is useful at this point to introduce a number of definitions related 
to climate and climatic change. In what may be called the "common" 
definition, climate is the average of the various weather elements, usually 
taken over a particular 30-year period. A more useful definition is 
what we shall call the "practical" definition, which introduces the con- 
cept of a climatic state. This and related definitions are as follows: 

Climatic state. This is defined as the average (together with the 
variability and other statistics) of the complete set of atmospheric, 
hydrospheric, and cryospheric variables over a specified period of time 
in a specified domain of the earth-atmosphere system. The time interval 
is understood to be considerably longer than the life span of individual 
synoptic weather systems (of the order of several days) and longer than 
the theoretical time limit over which the behavior of the atmosphere can 
be locally predicted (of the order of several weeks) . We may thus speak, 
for example, of monthly, seasonal, yearly, or decadal climatic states. 

Climatic variation. This is defined as the difference between climatic 
states of the same kind, as between two Januaries or between two 
decades. We may thus speak, for example, of monthly, seasonal, yearly, 
or decadal climatic variations in a precise way. The phrase "climatic 
change" is used in a more general fashion but is generally synonymous 
with this definition. 

Climatic anomaly. This we define as the deviation of a particular 
climatic state from the average of a (relatively) large number of 
climatic states of the same kind. We may thus speak, for example, of 
the climatic anomaly represented by a particular January or by a par- 
ticular year. 

Climatic variability. This we define as the variance among a number 
of climatic states of the same kind. We may thus speak, for example, 
of monthly, seasonal, yearly, or decadal climatic variability. Although 
it may be confusing, this definition of climatic variability includes the 
variance of the variability of the individual climatic states. 

The foregoing definitions are useful for two reasons. First, the con- 
cept of climatic state preserves the essence of what is usually connoted 
by climate, while circumventing troublesome problems of statistical 



20 UNDERSTANDING CLIMATIC CHANGE 

stability. Second, climatic states represent definite realizations or 
samples of climate (rather than the climate per se) and are comparable 
with the climates simulated by numerical general circulation experi- 
ments. There are many other definitions in existence to distinguish 
particular statistical characteristics of climate and climatic change 
(such as climatic fluctuations, oscillations, periods, cycles, trends, and 
rhythms). The above definitions are generally adequate for our pur- 
poses, although we shall later consider another definition of climate 
related to the climatic system. We shall also subsequently introduce 
the concepts of climatic noise and climatic predictability. Except when 
otherwise indicated, the use of the word "climate" in this report is to 
be considered an abbreviation for climatic state. 

It should be noted that we have included the oceans in the definition 
of a climatic state, as well as information on other aspects of the physical 
environment. The ensemble of statistics required to completely describe 
a climatic state is presently available for only a few regions and for 
limited periods of time. The climatic data-analysis and monitoring 
programs recommended in Chapter 6 are intended to fill in as much of 
the gap as possible with available data and to ensure that at least certain 
critical data are systematically gathered for an extended period of time 
in the future. 



CAUSES OF CLIMATIC CHANGE 

While the above discussion may describe the processes responsible for 
the maintenance of climate, it is an inadequate description of the 
processes involved in climatic change. Here we are on less secure 
ground and must consider a wide range of possible interactions among 
the elements of the climatic system. It is these interactions that are 
responsible for the complexity of climatic variation. 

Climatic Boundary Conditions 

If we view the gaseous, liquid, and ice envelopes surrounding the 
earth as the internal climatic system, we may regard the underlying 
ground and the space surrounding the earth as the external system. The 
boundary conditions then consist of the configuration of the earth's 
crust and the state of the sun itself. Changes in these conditions can 
obviously alter the state of the climatic system, i.e., they can be causes 
of climatic variation. 

Each of the external processes illustrated in Figure 3.1 may be used 
to develop a climatic theory, on which basis one may attempt to explain 



PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 21 

certain features of the observed climatic changes. For example, changes 
of the distribution of solar radiation have been used since the time of 
Milankovitch (1930) to explain the major glacial-interglacial cycles of 
the order of 10 l to 10 s years. Aside from the question of variations of 
the sun's radiative output, variations of the earth's orbital parameters 
produce changes in the intensity and geographical pattern of the seasonal 
and annual radiation received at the top of the atmosphere and in the 
length of the radiational seasons in each hemisphere. These effects, 
which are known with considerable accuracy, have resulted in occasional 
variations of the seasonal insolation regime several times larger than 
those now experienced. These orbital elements (eccentricity, obliquity, 
and precession) vary with periods averaging about 96,000 years, 41,000 
years, and 21,000 years, respectively. Because the seasons themselves 
represent substantial* climatic variations, such astronomical theories of 
climatic change must be given careful consideration. 

The separate question of the climatic effects of possible changes in 
the sun's radiation (i.e., changes of the so-called solar constant) has a 
much less firm physical basis. Not only are the measured short-period 
variations of solar output quite small, but the repeated search for 
climatic periodicities linked with the 11 -year and 80-year sunspot cycles 
has not yielded statistically conclusive results. The question of still 
longer-period solar variations cannot be adequately examined with 
present data, although over periods of the order of hundreds of millions 
of years the sun's radiation seems likely to have changed. The time 
range of this and other possible causative factors of climatic change is 
shown in Figure 3.3. 

On time scales of tens of millions of years there are changes in the 
shapes of the ocean basins and the distribution of continents as a 
result of sea-floor spreading and continental drift (see Figure 3.3). 
Over geological time, these processes must have resulted in substantial 
changes of global climate. Just how much of the recorded paleoclimatic 
variations may eventually be accounted for by such effects, however, is 
not known, and applying climatic models to the systematic reconstruc- 
tion of the earth's climatic history prior to about 10 million years ago 
is an important component of the research program recommended in this 
report (see Chapter 6). In such climatic reconstructions, the oceans 
must be simulated along with the atmosphere, and eventually the ice 
masses must also be reproduced. Accompanying the migration of the 
land masses are the processes of mountain building, epeirogeny, iso- 
static adjustment, and sea-level changes, all of which must also be taken 
into account. 

Yet another external cause of climatic variation is the changes in 











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PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 23 

the composition of the atmosphere resulting from the natural chemical 
evolution of the nitrogen, oxygen, and carbon dioxide content in re- 
sponse to geological and biological processes, as well as from the efflu- 
ents of volcanic eruptions. On shorter time scales, however, it is prob- 
ably the injection of dust particles into the atmosphere by volcanoes that 
has produced a more significant climatic effect by modifying the at- 
mospheric radiation balance (see Figure 3.2). The progressive enrich- 
ment of the atmospheric C0 2 content, which has occurred during this 
century as a result of man's combustion of fossil fuels (amounting to an 
increase of order 10 percent since the 1880's), must also be con- 
sidered an external cause of climatic variation. 

These considerations lead to the "physical" definition of climate 
as the equilibrium statistical state reached by the elements of the at- 
mosphere, hydrosphere, and cryosphere under a set of given and fixed 
external boundary conditions. There is, of course, the possibility that a 
true equilibrium may not be reached in a finite time due to the disparity 
of the response times of the system's components, but this is neverthe- 
less a useful definition. By progressively reducing the internal climatic 
system to include only the atmosphere and ocean (in equilibrium with 
the land and ice distribution ) , and then to include only the atmosphere 
itself (in equilibrium with the ocean, ice, and land), a hierarchy of 
climates may be defined which is useful for the analysis of questions of 
climatic determinism. 

Climatic Change Processes and Feedback Mechanisms 

Important as the above processes may be for the longer-period varia- 
tions of climate, there are other factors that may also produce climatic 
change. These involve changes in the large-scale distribution of the 
effective internal driving mechanisms for the atmosphere and ocean. 

Variations of the global ice distribution, for example, have a sig- 
nificant effect on the net heating of the atmosphere (by virtue of the 
ice's effective control of the surface heat budget), and thereby may 
change the meridional heating gradient that drives the atmospheric 
(and oceanic) circulation. An equally significant change (for the 
oceans, at least) may be introduced by widespread salinity variations, 
as caused, for example, by the melting of ice. The salinity of the ocean 
surface water is in turn closely related to the formation of relatively 
dense bottom water, which by sinking and spreading fills the bulk of 
the world's ocean basins. 

Such processes may act as internal controls of the climatic system, 



24 UNDERSTANDING CLIMATIC CHANGE 

with time scales extending from fractions of a year to hundreds and 
even thousands of years (see Figure 3.3). Some of these processes dis- 
play a coupling or mutual compensation among two (or more) elements 
of the internal climatic system. Such interactions or feedback mecha- 
nisms may act either to amplify the value or anomaly of one of the 
interacting elements (positive feedback) or to damp it (negative feed- 
back). Because of the large number of degrees of freedom of the 
ocean-atmosphere system (for the moment considering the ice distribu- 
tion to be fixed), there are a large number of possible feedback 
mechanisms within the ocean, within the atmosphere, and between the 
ocean and the atmosphere. These same degrees of freedom, however, 
invite a high risk of error in any qualitative analysis, and in some cases 
equally plausible arguments of this sort lead to opposite conclusions. 

Some of the more prominent feedback effects operate among the 
shorter-period processes of climatic change, especially those concerning 
the radiation balance over land and the energy balance over the ocean. 
For example, a perturbation of the ocean-surface temperature may 
modify the transfer of sensible heat to the overlying atmosphere, and 
thereby affect the atmospheric circulation and cloudiness. These changes 
may in turn affect the ocean-surface temperatures through changes in 
radiation, wind-induced mixing, advection, and convergence (and may 
subsequently affect the deep-ocean temperatures through geostrophic 
adjustment to the convergence in the boundary layer). These processes 
may result either in the enhancement or reduction of the initial anomaly 
of sea-surface temperature. A number of studies have shown positive 
feedback of this sort for several years' time in the North Pacific Ocean. 

The greenhouse effect (in which the absorption of long- wave radiation 
by water vapor produces a higher surface temperature), is probably the 
best known example of a semipermanent positive feedback process, al- 
though other positive feedbacks of climatic importance may be noted. 
One of these is the snow cover-albedo-temperature feedback, in which 
an increase of snow extent increases the surface albedo and thereby 
lowers the surface temperature. This in turn (all else being equal) 
further increases the extent of the snow cover. An example of negative 
feedback is the coupling between cloudiness and surface temperature 
noted earlier. In this scheme, an initial increase of surface temperature 
serves to increase the evaporation, which is followed by an increase 
of cloudiness. This in turn reduces the solar radiation reaching the 
surface and thereby lowers the initial temperature anomaly. Here we 
have ignored the effects of long-wave radiation and of advective pro- 
cesses in both ocean and atmosphere, but these examples serve to illus- 
trate the uncertainty that must be attached to such arguments. 



PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 25 

While there is much evidence to support the existence of feedback 
processes, the key phrase in their qualitative use is "all else being equal." 
In a system as complex as climate, this is usually not the case, and an 
anomaly in one part of the system may be expected to set off a whole 
series of adjustments, depending on the type, location, and magnitude 
of the disturbance. Any positive feedback must, in any event, be 
checked at some level by the intervention of other internal adjustment 
processes, or the climate would exhibit a runaway behavior. We do not 
adequately understand these adjustment mechanisms, and their system- 
atic quantitative exploration by numerical climate models is an 
important task for the future (see Chapter 6). In that research it will 
be essential to use coupled models of atmosphere and ocean, and these 
must be calibrated with great care so as not to distort the feedback 
mechanisms themselves. 



Climatic Noise 

Climatic states have been defined in terms of finite time averages and 
as such are subject to fluctuations of statistical origin in addition to the 
changes of a physical nature already discussed. Since these statistical 
fluctuations arise from the day-to-day fluctuations in weather (the 
autovariation of the atmosphere identified in Figure 3.3), they are un- 
predictable over time scales of climatological interest and are therefore 
appropriately defined as "climatic noise." The amplitude of this noise 
decreases approximately as the square root of the length of the time- 
averaging interval, but some remains at any finite time scale (Leith, 
1973; Chervin et at, 1974). A key problem of climatic variation on any 
time scale is therefore the determination of the "climatic predictability," 
which we may define as the ratio of the magnitude of the potentially 
predictable climatic change of physical origin to the magnitude of this 
unpredictable climatic noise. 

ROLE OF THE OCEANS IN CLIMATIC CHANGE 

It has been noted that the oceans play a prominent role in the determina- 
tion of climate through the processes at the air-sea interface that govern 
the exchanges of heat, moisture, and momentum. While these condi- 
tions are actually determined mutually by the atmosphere and the 
ocean, they are likely dominated by the ocean on at least the longer 
climatic time scales. It is the high thermal and mechanical oceanic 
inertia that requires that special consideration be given to the role of 
the ocean in climatic change. 



26 UNDERSTANDING CLIMATIC CHANGE 

Physical Processes in the Ocean 

Over half of the solar radiation reaching the earth's surface is absorbed 
by the sea. This solar radiation, along with the surface wind stress, is the 
ultimate energy source for a variety of physical processes in the ocean 
whose climatic importance is essentially a function of their time scales. 
The absorption of solar radiation is primarily responsible for the exist- 
ence of a warm surface mixed layer of order 10 2 m deep found over 
most of the world's oceans. This warm surface layer represents a large 
reservoir of heat and acts as a significant thermodynamic constraint on 
the atmospheric circulation. 

The exchange of the ocean's heat with the atmosphere occurs over a 
wide range of time scales and largely determines the relative importance 
of other physical processes in the ocean for climatic change. Some of this 
heat is used for surface evaporation, some is stored in the surface layer, 
and some is moved downward into deeper water by various dynamical 
and thermodynamical processes. The fluxes of latent and sensible heat 
into the atmosphere are commonly parameterized in atmospheric models 
as functions of the large-scale surface wind speed and the vertical 
gradients of humidity and temperature in the air over the ocean surface. 
These fluxes are actually accomplished by small-scale turbulent proc- 
esses in the surface boundary layer whose behavior is not adequately 
understood. Physical processes in the ocean such as vertical convective 
motions (depending on the local vertical stratification of temperature 
and salinity) and wind-induced stirring also affect the depth and struc- 
ture of the mixed layer, as shown, for example, by the simulations of 
daily variations of local mixed layer depth by Denman and Miyake 
(1973). Other small-scale processes such as salt fingering and internal 
waves also produce transports that may contribute significantly to the 
overall vertical mixing in the ocean. Therefore, the dynamics of the 
ocean's surface layer must be taken into account in even the simplest 
of climate models. 

It is becoming apparent that the most energetic motion scale in the 
oceans is that of the mesoscale eddy, whose period is of the order of a 
few months and whose horizontal wavelength is of the order of several 
hundred kilometers. The kinetic energy of these motions, which is pre- 
dominantly in the barotropic and first baroclinic vertical mode, may be 
one or two orders of magnitude greater than that of the time-averaged 
motions themselves. In a general sense, these slowly evolving eddies are 
the counterpart of the larger-scale transient cyclones and anticyclones in 
the atmosphere. An understanding of the physical processes responsible 
for the origin and behavior of these eddies and their role in the oceanic 



PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 27 

general circulation is essential for further insight into the dynamics of 
the vast open ocean regions. 

In addition to the surface interactions, vertical mixing processes, and 
mesoscale motions, the study of the longer-period variations of climate 
clearly requires consideration of the large-scale dynamics of the com- 
plete oceanic circulation. This includes the large-scale pattern of wind- 
driven and thermohaline currents and their associated horizontal and 
vertical transports of heat, momentum, and salt. Of particular im- 
portance here is the study of the local dynamics of the intense bound- 
ary and equatorial currents and the relative roles of inertial and topo- 
graphic influences. The characteristic variations of these large-scale 
processes are on time scales of the order of seasons and years in the 
near-surface waters but may occur in progressively longer time scales 
in deeper water. The longest oceanic adjustment time associated with 
the "permanent" ocean circulation is of the order 10 3 years (see Figure 
3.3). For climatic variations on these time scales, therefore, the entire 
water mass of the global ocean must be taken into account. 

Modeling the Oceanic Circulation 

The systematic examination of the various mechanisms and feedbacks 
by which the oceanic thermal structure and circulation are maintained 
on various time scales is largely a task for the future. In this research, 
it will be necessary to conduct intensive observational programs in order 
to gain greater understanding of the various oceanic physical processes 
themselves and to construct numerical models of the oceanic circulation 
in which these processes are correctly represented. 

For climatic studies, it is important that the heat and energy balances 
of the ocean be modeled correctly over the time and space scales of 
interest, and this cannot now be said to have been achieved. The classical 
ocean circulation models, which were initiated in the late 1940's and 
further developed in the following decades, do account for the gross 
features of the ocean circulation, such as the major current systems and 
the large-scale oceanic thermal structure (see Appendix B). But even 
these features are physically and geometrically distorted by the con- 
sideration of only the larger-scale, relatively viscous motions. The 
commonly used vertical thermal eddy diffusivity in such models is 
also questionable and may be an order of magnitude too high, as in- 
dicated, for example, by recent studies on oceanic tritium concentra- 
tions. This alone will produce a distortion of the processes responsible 
for deep-water formation in such models. 

But perhaps more important is the fact that numerical ocean models 



28 UNDERSTANDING CLIMATIC CHANGE 

have not had a sufficiently fine horizontal resolution to portray the 
mesoscale eddies, either in the open ocean or in the restricted regions 
of concentrated currents. The accuracy with which the meandering and 
vortex shedding of boundary currents such as the Gulf Stream or 
Kuroshio must be modeled, or the resolution required for the transient 
behavior of the equatorial and Antarctic current systems, depends on 
the extent to which these features are coupled to the semipermanent or 
primary current systems themselves and on the time scales under con- 
sideration. It is unlikely, however, that these features, or the mesoscale 
eddies, can be successfully modeled with constant eddy diffusion 
coefficients. 

To study the role of the oceans in climatic change, it is necessary 
to construct dynamically and energetically correct oceanic general cir- 
culation models and to couple them, in appropriate versions, to 
similarly accurate and compatible atmospheric models. Some experi- 
ence with simplified coupled models of coarse resolution has already 
been gained, as discussed in Appendix B. Further tests of coupled 
models are necessary in which the oceanic mesoscale eddies are re- 
solved, in order that we may understand their role in the oceanic heat 
balance and their relationship to the climatically important changes of 
sea-surface temperature. Since computational limitations will likely 
preclude the resolution of these eddies throughout the world ocean, 
their successful parameterization will become an important problem for 
future research. 

Of particular importance for climate studies is the construction of an 
accurate model of the oceanic-surface mixed layer, since all the physical 
processes in the ocean ultimately exert their influence on the atmosphere 
through the surface of the sea. Until the dynamics of this oceanic 
boundary layer are better understood, our ability to model climatic 
variations on any time scale will remain seriously limited. 

SIMULATION AND PREDICTABILITY OF CLIMATIC VARIATION 

Climate Modeling Problem 

From the above remarks it is clear that the problem of modeling climatic 
variation is fundamentally one of constructing a hierarchy of coupled 
atmosphere-ocean models, each suited to the physical processes domi- 
nant on a particular time scale. The attack on this problem is now in its 
infancy. Whether we consider changes of the external boundary con- 
ditions or changes of the internally controlled physical processes and 
feedback mechanisms, we note from Figure 3.3 the wide range of time 



PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 29 

intervals over which characteristic climatic events occur and that many 
of these involve interactions among the atmosphere, oceans, ice, and 
land. Because of the system's nonlinearity, we may expect a broad 
range of response in both space and time in the individual climatic 
variables. This is just what the climatic record shows. 

To study the relative contribution of individual physical processes to 
the overall "equilibrium" climatic state, one approach is to test the 
sensitivity of the statistics generated by a climate model to perturbations 
in the parameters that influence that particular physical process. In such 
a modeling program, the effects of changes can first be tested in isolation 
from other interacting components of the system and then in concert 
with all known processes in a complete climatic model. In this research, 
we should not rely exclusively on the general circulation models (gcm's) 
but should employ a. variety of modeling approaches. We note, however, 
that not only are the gcm's (and the coupled gcm's in particular) 
useful in the calibration of the simpler models, but they are essential to 
the detailed diagnosis of the shorter-period climatic states that are in 
approximate statistical equilibrium with slowly changing boundary 
conditions. 

A fundamental approach to the problem of modeling climate and 
climatic variation must proceed through the consideration of dynamical 
models of the coupled components of the climatic system. In minimum 
practical terms, this means the joint atmosphere-ocean system, although 
for some purposes (such as the behavior of ice sheets and glaciers) the 
cryosphere must be included as well. Efforts to assemble such models 
are just getting under way, and their further development is given high 
priority in the research program recommended in Chapter 6. 

Predictability and the Question of Transitivity 

It is possible to regard climatic change as a conventional initial/ 
boundary-value problem in fluid dynamics, if we define the climatic sys- 
tem as consisting of the atmosphere, hydrosphere, and cryosphere. In 
this deterministic view the behavior of the system is governed by the 
changes of the external boundary conditions (see Figure 3.1). Over 
relatively short periods, it is even possible to regard the land ice masses 
as part of the external conditions as well. It is probably not possible, 
however, to remove the hydrosphere from the internal system and still 
talk meaningfully about climatic variation, as the surface layers of the 
ocean interact with the atmosphere on the shortest time scales associated 
with climate (see Figure 3.3). Decoupling of the ocean, however, is 
exactly what has so far been done in conventional atmospheric and 



30 UNDERSTANDING CLIMATIC CHANGE 

oceanic general circulation models, although preliminary efforts to 
consider the coupled system have been made (see Appendix B). 

Even with the atmosphere (together with certain surface effects) 
regarded as the sole component of the climatic system, and with all ex- 
ternal boundary conditions held fixed, there is, in spite of our physical 
expectations, no assurance that there will be a climate in the sense that 
time series generated by the atmospheric changes will settle into a 
statistically steady state; and no assurance that the climate, if it exists, is 
unique in the sense that the statistics are independent of the initial state. 
It is therefore useful to define a random time series (or the system 
generating such a series) as transitive if its statistics (and hence its 
climatic states) are stable and independent of the initial conditions and 
as intransitive if not. As shown by Lorenz (1968), nonlinear systems, 
which are far simpler than the atmosphere, sometimes display a ten- 
dency to fluctuate in an irregular manner between two (or more) in- 
ternal states, while the external boundary conditions remain completely 
unchanged. This behavior is related to the system's transitivity and is 
illustrated in Figure 3.4. 

Let us assume that two different states of a climatic system are 
possible at a time f=0, such as A and B in Figure 3.4, and let us con- 
sider that A is the climatic state that would normally be "expected" 
under the given constant boundary condition. In a completely transitive 
system, the climatic state B would approach the state A with the 
passage of time and eventually become indistinguishable from it. This 
would correspond to a unique solution for the climate under fixed 
boundary conditions. In a completely intransitive system, on the other 
hand, the climatic state B would remain unchanged, and two possible 
solutions would exist. There would in this case, moreover, be no way 
in which we could continue to identify the state A as the "normal" or 
correct solution, as state B would presumably furnish an equally 
acceptable set of climatic statistics. 

A third behavior, however, is perhaps the most interesting of all, and 
is displayed by an almost-intransitive system. In this case, the system in 
state B may behave for a while as though it were intransitive, and then 
at time t x shift toward an alternate climatic state A, where it might re- 
main for a further period of time. At time t 2 the system might then return 
to the original climatic state B, where it could remain or enter into 
further excursions. The climate exhibited by such a system would thus 
consist of two (or more) quasi-stable states, together with periods 
of transition between them. For longer periods of time the system might 
have stable statistics, but for shorter periods of time it would appear to 
be intransitive. 



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32 UNDERSTANDING CLIMATIC CHANGE 

Because the atmosphere is constantly subject to disturbances, such as 
those arising from flow over rough terrain or from the occurrence of 
baroclinic instability, one might think that it could not be an almost- 
intransitive system and fail to show greater excursions of annual and 
decadal climatic states than it does. This depends, of course, on the level 
of variability associated with individual climatic states and hence on the 
time interval we select to define the climatic state itself (and on how 
close neighboring quasi-stable states might be). What may appear to be 
a climatic transition on one time scale may become the natural noise 
level of a climatic state defined over a longer interval. This is consistent 
with many of the climatic records presented in Appendix A. 

Even so, it might still be possible for the coupled ocean-atmosphere 
system or for the coupled ocean-atmosphere-ice system to be almost 
intransitive. One cannot help but be struck by the appearance of those 
proxy records that display repeated transitions between two states (see 
Figures A. 13 and A. 14 in particular). This evidence suggests that the 
glacial-interglacial oscillations that have characterized the past million 
years of the earth's climatic history may be the climatic transitions of 
an almost-intransitive system. Another possible example of this 
phenomenon is the irregular and relatively sudden reversal of the earth's 
magnetic poles. The search for further evidence of this sort in both the 
paleoclimatic record and in the climatic history generated by numerical 
models is an important task for future research. 

As though the specter of almost-intransitivity were not enough, on 
the longer-time scales of climatic variation it is equally important to 
recognize another, potentially serious complication. If it turns out that 
climatic evolution is influenced to a significant degree by environmental 
impacts originating outside the atmosphere-ocean-cryosphere system, 
then the predictability of climate will be additionally constrained by the 
predictability of the environment in a larger sense. This, in turn, could 
turn out to be the greatest stumbling block of all, as illustrated, for 
example, by the difficulty of predicting the timing and intensity of vol- 
canic eruptions (which inject radiation-attenuating layers of dust into 
the upper atmosphere) and by the difficulty of predicting the behavior 
of the sun itself, which is the ultimate source of the energy driving the 
climatic system. 

As noted earlier, the predictability of climatic variation is con- 
strained by an inherent limitation in the detailed predicability of the 
atmosphere and ocean. Climatic noise as previously defined thus arises 
from the unavoidable uncertainty in the determination of the initial 
state and from the nonlinear nature of the relevant dynamics, as shown, 
for example, by Lorenz (1969). Fluctuations in the weather for periods 



PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE 33 

beyond a few weeks may therefore be treated in large part as though 
they were generated by an unpredictable random process. The observed 
time series of many meteorological variables may be reasonably well 
modeled by a first-order Markov process with a time (r) -lagged cor- 
relation given by R(t) =exp(-v|r|), with a constant decay rate v of the 
order of 0.3 day 1 . The corresponding power spectrum as a function of 
frequency cu is given by P(o>) =/4/(v 2 +<» 2 ), where A is a constant. 
As <a— »0 for such a spectrum, we have P(o>)->A/v 2 , which is a constant, 
and the very-low-frequency end of the spectrum therefore appears 
"white." There is thus some contribution to climatic variations on all 
time scales, no matter how long, arising from the fluctuations of the 
weather. 

While these considerations do not directly address the physical basis 
of climatic change,, they are nevertheless basic to our view of the pre- 
dictability of climatic change. What parts of climatic variations on 
various time scales are potentially predictable, and what parts are just 
climatic noise? In the power spectrum is there potentially predictable 
variability above the "white," low-frequency end of the daily weather 
fluctuations, or is it possible that some of the long-term compensation 
processes, such as those shown in Figure 3.3, might depress the spec- 
trum below its white extension to o> = 0? 

The 250-year record of monthly mean temperatures in central 
England compiled by Manley (1959) shows small lagged correlations 
significantly above that of weather out to about 6 months, small but 
perhaps significant lagged correlations at 2 and 4 years, and a generally 
white spectrum with some evidence of extra variability for periods of a 
few decades and longer (C. E. Leith, ncar, Boulder, Colorado, un- 
published results). The 6-month lagged correlation may well be a re- 
flection of the role of North Atlantic sea-surface temperature anomalies 
on the English climate and illustrates the somewhat longer periods of 
the autovariation of the coupled ocean-atmosphere system over those 
of the atmosphere alone, as indicated in Figure 3.3. Additional evidence 
of even longer-period variability is found in the historical and paleo- 
climatic records (Kutzbach and Bryson, 1974; see Figure A. 5). Further 
studies of this kind should be made with statistical tests not only of the 
pessimistic null hypothesis that nothing is predictable but also of 
hypotheses that are framed more optimistically. 

Long-Range or Climatic Forecasting 

As our understanding of the physical basis of climatic variation grows, 
we hope to be able to discern the predictable climatic change signal 



34 UNDERSTANDING CLIMATIC CHANGE 

from the unpredictable climatic noise and to describe with some con- 
fidence the character of both past and likely future climates. In view 
of the questions posed by limited predictability, however, this discern- 
ment may be limited to those circumstances in which there is a relatively 
large change in the processes or boundary conditions of the climatic 
system. The related problem of forecasting specific seasonal and annual 
climatic variations rests upon the same physical basis and may prove 
more difficult to solve. To reach these goals will require the coordinated 
use of all our research tools, whether they be observational, numerical, 
or theoretical. The capstone of these efforts will be the emergence of an 
increasingly well-defined and tested theory of climatic variation. 

Whether the predictability of climatic change turns out to be lower 
than many would like to believe or to be limited to a finite range as in 
the problem of weather forecasting, the quest for understanding must be 
made. Our recommendations for the research that we believe to be 
a necessary part of this effort are presented in detail in Chapter 6. 



4 



PAST CLIMATIC VARIATIONS AND THE 
PROJECTION OF FUTURE CLIMATES 



It is universally accepted that global climate has undergone significant 
variations on a wide variety of time scales, and we have every reason to 
expect that such variations will continue in the future. The development 
of an ability to forecast these future variations, even on time scales as 
short as one or two decades, is an important and challenging task. Study 
of the instrumental, historical, and paleoclimatic records not only offers 
a basis for projection into the future but furnishes insight into the 
regional effects of global climatic changes. This chapter attempts to 
summarize our knowledge of past climatic variations and to give some 
indication of the further research that must be carried out on critical 
aspects of this subject. Further details of the record of past climates are 
given in Appendix A. 

IMPORTANCE OF STUDIES OF PAST CLIMATES 

In order to understand fully the physical basis of climate and climatic 
variation, we must examine the earth's atmosphere-ocean-ice system 
under as wide a range of conditions as possible. Most of our notions of 
how the climatic system works, and the tuning of our empirical and 
dynamical models, are based on observations of today's climate. In order 
that these ideas and models may be useful in the projection of future 
climates, it is necessary that they be calibrated under as wide a range 
of conditions as possible. The only documented evidence we have of 
climates under boundary conditions significantly different from today's 

35 



36 UNDERSTANDING CLIMATIC CHANGE 

comes from the paleoclimatic record. It is here that paleoclimatology, in 
conjunction with climatic modeling, can make an especially valuable 
contribution to the resolution of the problem of climatic variation. 

Modern instrumental data suggest that the atmosphere, at least, may 
be capable of assuming quite different circulation patterns even with 
relatively constant boundary conditions and that the resulting variability 
of climate is strongly dependent on geographical location. Although the 
data base is much less complete for the oceans, persistent anomalies of 
sea-surface temperature appear to be related to atmospheric circulation 
regimes over time scales of months and seasons, and the oceans may 
show other longer-period variations of which we are now unaware. 

In general, the record of past climate indicates that the longer the 
available record, the more extreme are the apparent climatic variations. 
An immediate consequence of this "red-noise" characteristic is that the 
largest climatic changes are not revealed by the relatively short record 
of instrumental observation but must instead be sought through paleo- 
climatic studies. The record of past climates also contains important 
information on the range of climatic variability, the mean recurrence 
interval of rare climatic events, and the tendency for systematic time- 
wise behavior or periodicity. Such climatic characteristics are in general 
shown poorly, if at all, by the available instrumental records. 

RECORD OF INSTRUMENTALLY OBSERVED CLIMATIC CHANGES 

Our knowledge of instrumentally recorded climatic variations is largely 
confined to the record of the past two centuries or so, and it is only in 
the last 100 years that synoptic coverage has permitted the analysis of 
the geographical patterns of climatic change over large portions of the 
globe. It is only during the past 25 years or so that systematic observa- 
tions of the free atmosphere (mainly in the northern hemisphere) have 
been made and that regular measurements of the ocean surface waters 
have been available in even limited regions. Enough data have been 
gathered, however, to permit the following summary. 

A striking feature of the instrumental record is the behavior of tem- 
perature worldwide. As shown by Mitchell (1970), the average surface 
air temperature in the northern hemisphere increased from the 1880's 
until about 1940 and has been decreasing thereafter (see Figure A. 6, 
Appendix A). Starr and Oort (1973) have reported that, during the 
period 1958-1963, the hemisphere's (mass-weighted) mean tempera- 
ture decreased by about 0.6 °C. In that period the polar and subtropical 
arid regions experienced the greatest cooling. The cause of this variation 
is not known, although clearly this trend cannot continue indefinitely. 



PAST CLIMATIC VARIATIONS— PROJECTION OF FUTURE CLIMATES 37 

It may represent a portion of a longer-period climatic oscillation, al- 
though statistical analysis of available records has failed to establish any 
significant periodic variation between the quasi-biennial cycle and 
periods of the order of 100 years. The corresponding patterns of pre- 
cipitation, cloudiness, and snow cover have not been adequately deter- 
mined, and it would be of great interest to examine the simultaneous 
variations of oceanic heat storage and imbalances of the planetary radia- 
tion budget, once the necessary satellite observations become available. 
For the earlier instrumental period, there are scattered records of 
temperature, rainfall, and ice extent, which clearly show individual 
years and decades of anomalous character. The only apparent trend is 
a gradual warming in the European area since the so-called Little Ice 
Age of the sixteenth to nineteenth centuries. 

HISTORICAL AND PALEOCLIMATIC RECORD 

Two sources of data are available to extend the record of climate into 
the pre-instrumental era: historical sources, such as written records, 
and qualitative observations, which give rise to what may be called 
"historical" climatic data; and various natural paleoclimatic recorders, 
which give rise to what may be called "proxy" climatic data. 



Nature of the Evidence 









The historical record contains much information relating to climate 
and climatic variation over the past several hundred to several thousand 
years, and this information should be located, cataloged, and evaluated. 
Historical data on crop yields, droughts, and winter severity from 
manuscripts, explorations, and other sources sometimes provide the 
only available information on the general character of the climate of 
the historical past. Such information is especially useful in conjunction 
with selected tree-ring, ice core, and lake sediment data in diagnostic 
studies of the higher-frequency climatic variability on the time scales 
of years, decades, and centuries. 

For earlier periods, the paleoclimatic record becomes increasingly 
fragmentary and ultimately nil for the oldest geological periods. But for 
the past million years, and especially for the past 100,000 years, the 
paleoclimatic record is relatively continuous and can be made to yield 
quantitative estimates of the values of a number of significant climatic 
parameters. Each record, however, must first be calibrated or processed 
to provide an estimate of the climate. The elevation of an ancient coral 
reef, for example, is a record of a previous sea level, but before it can 



38 UNDERSTANDING CLIMATIC CHANGE 

be used for paleoclimatic purposes the effect of local crustal movements 
must be removed. The taxonomic composition of fossil assemblages in 
marine sediments and the width of tree rings, for example, are known 
to reflect the joint influence of several ecological factors; here multi- 
variate statistical techniques can be used to obtain estimates of selected 
paleoclimatic parameters such as temperature and precipitation. 

In order to be useful, a proxy data source must also have a strati- 
graphic character; that is, the ambient values of a climatically sensitive 
parameter must be preserved within the layers of a slowly accumulating 
natural deposit or material. Such sources include the sediments left by 
melting glaciers on land; sediments in peat bogs, lakes, and on the 
ocean bottom; the layers in soil and polar ice caps; and the annual 
layers of wood formed in growing trees. Since no proxy source yields as 
long and continuous a record as would be desired, and the quality of 
data varies considerably from site to site, a coherent picture of past 
climate requires the assembly of data from different periods and with 
different sampling intervals. Such characteristics of the principal proxy 
data sources are summarized in Table A.l of Appendix A. 

After proxy data have been processed and stratigraphically screened, 
an absolute chronology must be established in order to date specific 
features in the climatic record. The most accurate dating technique is 
that used in tree-ring analysis, where dates accurate to within a single 
year may be determined over the past several thousand years under 
favorable conditions. Annually layered lake sediments and the younger 
ice cores also have a potential dating accuracy to within several years 
over the past few millenia. For suitable materials, 14 C-dating methods 
extend the absolute time scale to about 40,000 years, with an accuracy 
of about 5 percent of the material's true age. Beyond the range of 14 C 
dating, the analysis of the daughter products of uranium decay make 
possible the reconstruction of the climatic chronology of the past million 
years. For even older records, our chronology is based primarily on 
potassium-argon radiometric dating as applied to terrestrial lava and 
ash beds. Stratigraphic levels dated by this method are then correlated 
with undated sedimentary sequences by the use of paleomagnetic 
reversals and characteristic floral and faunal boundaries. 



Summary of Paleoclimatic History 

From the overview of the geological time scale, we live in an unusual 
epoch: today the polar regions have large ice caps, whereas during most 
of the earth's history the poles have been ice-free. As shown in Figure 
A. 15 of Appendix A, only two other epochs of extensive continental 



PAST CLIMATIC VARIATIONS— PROJECTION OF FUTURE CLIMATES 39 

glaciation have been recorded, one during late Precambrian time (ap- 
proximately 600 million years ago) and one during Permo-Carbonif- 
erous time (approximately 300 million years ago). During the era that 
followed the Permo-Carboniferous ice age, the earth's climates returned 
to a generally warmer, nonglacial regime. 

Before the end of the Mesozoic era (approximately 65 million years 
ago) climates were substantially warmer than today. At that time, the 
configuration of the continents and shallow ocean ridges served to 
block a circumpolar ocean current in the southern hemisphere. This 
barrier was formed by South America and Antarctica, which lay in 
approximately their present latitudinal positions, and by Australia, 
then a northeastward extension of Antarctica. About 50 million years 
ago, the Antarctic-Australian passage opened as Australia moved 
northeastward and- as the Indian Ocean widened and deepened. By 
about 30 million years ago, the Antarctic circumpolar current system 
was established, an event that may have decisively influenced the sub- 
sequent climatic history of the earth. 

About 55 million years ago global climate began a long cooling trend 
known as the Cenozoic climate decline (see Figure A.15). Approxi- 
mately 35 million years ago there is evidence from the marine record 
that the waters around the Antarctic continent underwent substantial 
cooling, and there is further evidence that about 25 million years ago 
glacial ice occurred along the edge of the Antarctic continent in some 
locations. During early Miocene time (approximately 20 million years 
ago) there is evidence that the low and middle latitudes were somewhat 
warmer. There is widespread evidence of further cooling about 10 
million years ago, including the growth of mountain glaciers in the 
northern hemisphere and substantial growth of the Antarctic ice sheet; 
this time may be taken as the beginning of the present glacial age. Evi- 
dence from marine sediments and from continental glacial features 
indicates that about 5 million years ago the already substantial ice sheets 
on Antarctica underwent rapid growth and even temporarily exceeded 
their present volume. Three million years ago continental ice sheets 
appeared for the first time in the northern hemisphere, occupying lands 
adjacent to the North Atlantic Ocean, and during at least the last 1 mil- 
lion years the ice cover on the Arctic Ocean was never significantly less 
than it is today. 

Once the polar ice caps formed, they began a long and complex series 
of fluctuations in size. Although the earlier record is still not clear, the 
last million years has witnessed fluctuations in the northern hemisphere 
ice sheets with a dominant period on the order of 100,000 years (see 
Figure A. 2). These fluctuations may have occurred in parallel with 



40 



UNDERSTANDING CLIMATIC CHANGE 



substantial changes in the volume of the West Antarctic ice sheet. By 
comparison, however, changes in the volume of the ice sheet in East 
Antarctica were quite small and were probably not synchronous with 
glaciations in the northern hemisphere. 

The major climatic events during the past 150,000 years were the 
occurrence of two glacial maxima of roughly equal intensity, one about 
135,000 years ago and the other between 14,000 and 22,000 years 
ago. Both were characterized by widespread glaciation and generally 
colder climates and were abruptly terminated by warm interglacial 
intervals that lasted on the order of 10,000 years. The penultimate 
interglacial reached its peak about 124,000 years ago, while the pres- 
ent interglacial (known as the Holocene) evidently had its thermal 
maximum about 6000 years ago. 

Between 22,000 and 14,000 years ago the northern hemisphere ice 
sheets attained their maximum extent (see Figure A. 24). The eastern 
part of the Laurentide ice sheet (which covered portions of eastern 
North America) and the Scandinavian ice sheet (which covered parts 
of northern Europe) both attained their maxima between 22,000 and 
18,000 years ago, several thousand years before the maximum of the 
Cordilleran ice sheet. About 14,000 years ago deglaciation began rather 
abruptly, and the Cordilleran sheet melted rapidly and was gone by 
10,000 years ago. The interval of deglaciation (14,000 to 7000 years 
ago) was marked in many places by significant secondary fluctuations 
about every 2000 to 3000 years. 

In general, the period about 7000 to 5000 years ago was warmer 
than today, although the records of mountain glaciers, tree lines, and 
tree rings reveal that the past 7000 years was punctuated in many parts 
of the world by colder intervals about every 2500 years, with the most 
recent occurring about 300 years ago. 

For the last 1000 years, the proxy records generally confirm the 
scattered observations in historical records. The cold period identified 
above is seen to have consisted of two periods of maximum cold, one 
in the fifteenth century and another in the late seventeenth century. The 
entire interval, from about 1430 to 1850, has long been referred to as 
the Little Ice Age and was characterized in Europe and North America 
by markedly colder climates than today's. 

INFERENCE OF FUTURE CLIMATES FROM PAST BEHAVIOR 

Notwithstanding the limitations of our present insight into the physical 
basis of climate, we are not altogether powerless to make certain in- 



PAST CLIMATIC VARIATIONS— PROJECTION OF FUTURE CLIMATES 41 

ferences about future climate. Beginning with the most conservative 
approach, we may use the climatic "normal" as a reference for future 
planning. In this approach, it is tacitly assumed that the future climate 
will mirror the recently observed past climate in terms of its statistical 
properties. Depending on the sensitivity of the climate-related applica- 
tion (and on the degree to which the climate is subject to change over 
a period of years following that for which the "normal" is defined), 
this kind of inference can be anything from highly useful to downright 
misleading. 

Of the various other approaches to the inference of future climate 
in which the attempt is made to capture more predictive information 
than is embodied in the "normal," the most popular have been those 
based on the supposition that climate varies in cycles. Since the develop- 
ment of modern techniques of time-series analysis, in particular those 
involving the determination of the variance (or power) spectrum, it 
has become clear that almost all alleged climatic cycles are either (1) 
artifacts of statistical sampling, (2) associated with such small fractions 
of the total variance that they are virtually useless for prediction pur- 
poses, or (3) a combination of both. Other approaches, developed to a 
high degree of sophistication in recent years, include several kinds of 
nonlinear regression analysis (in which no assumption need be made 
about the periodic behavior of the climatic time series), which appro- 
priately degenerate to a prediction of the "normal" in cases where the 
series possess no systematic temporal behavior. The full potential of such 
approaches is not yet clear but appears promising, at least in certain 
situations. 



Natural Climatic Variations 

Regardless of the approach taken to infer future climates, the view that 
climatic variation is a strictly random process in time can no longer be 
supported. It has been well established, for example, that many atmo- 
spheric variables are serially correlated on time scales of weeks, months, 
and even years. For the most part such correlations derive from "per- 
sistence" and resemble the behavior of a low-order Markov process. 
Unfortunately, nonrandomness of this kind does not lend itself to long- 
range statistical prediction. In addition to persistence, long-term trends 
have a tendency to show up in great number and variety in climato- 
logical time series (see Appendix A). Many such trends are now 
understood to originate from what are called inhomogeneities in the 
series, as, for example, effects of station relocations, changes in observ- 



42 UNDERSTANDING CLIMATIC CHANGE 

ing procedures, or local microclimatic disturbances irrelevant to large- 
scale climate. Even after statistical removal of such effects, many "real" 
trends nonetheless remain and may be recognized as part of a longer- 
term oscillation of climate. We must, moreover, recognize that the 
climatic record may also reflect various natural environmental disturb- 
ances, such as volcanic eruptions and perhaps changes of the sun's 
energy output, which are themselves only poorly predictable, if at all. 
Clearly, a climatic prediction based on the linear extrapolation into the 
future of a record containing such effects would be highly unrealistic. 

The behavior of longer climatic series is seemingly periodic, or quasi- 
periodic, especially those series that extend into the geological past as 
reconstructed from various proxy data sources. It is a fundamental 
problem of paleoclimatology to determine whether this behavior is 
really what it seems or whether it is an illusion created by the character- 
istic loss of high-frequency information due to the limited resolving 
power of most proxy climatic indicators. Illumination of this question 
would be of great importance to the determination of the basic causes of 
the glacial-interglacial climatic succession and to the assessment of 
where the earth stands today in relation to this sequence. 

Spectrum analyses of the time series of a wide variety of climatic 
indices have consistently displayed a "red-noise" character (see, for 
example, Gilman et al, 1963). That is, the spectra show a gradual in- 
crease of variance per unit frequency as one proceeds from high fre- 
quencies toward low frequencies. The lack of spectral "gaps" provides 
empirical confirmation of the lack of any obvious optimal averaging in- 
terval for defining climatic statistics. Most spectra of climatic indices 
are also consistent in displaying some form of quasi-biennial oscillation 
(see, for example, Brier, 1968; Angell et al., 1969; or Wagner, 1971). 
This fluctuation is most obvious in the wind data of the tropical strato- 
sphere but also has been shown to be a real if minor feature of the 
climate at the earth's surface as well. 

Time series of some of the longer instrumental records show some 
suggestion of very-low-frequency fluctuations (periods of about 80 years 
and longer), but the data sets are not long enough to establish the 
physical nature and historical continuity of such oscillations. While 
numerous investigators have reported spectral peaks corresponding to 
almost all intermediate periods, the lack of consistency between the 
various studies suggests that no example of quasi-cyclic climatic be- 
havior with wavelengths between those on the order of 100 years and 
the quasi-biennial oscillation have been unequivocally demonstrated on 
a global scale. Further discussion of these questions is given in Appendix 
A (p. 127 0). 






PAST CLIMATIC VARIATIONS— PROJECTION OF FUTURE CLIMATES 43 

Man's Impact on Climate 

While the natural variations of climate have been larger than those 
that may have been induced by human activities during the past century, 
the rapidity with which human impacts threaten to grow in the future, 
and increasingly to disturb the natural course of events, is a matter of 
concern. These impacts include man's changes of the atmospheric com- 
position and his direct interference with factors controlling the all- 
important heat balance. 



Carbon Dioxide and Aerosols 

The relative roles of changing carbon dioxide and particle loading as 
factors in climatic 'change have been assessed by Mitchell (1973a, 
1973b), who noted that these variable atmospheric constituents are not 
necessarily external parameters of the climatic system but may also be 
internal variables; for example, the changing capacity of the surface 
layers of the oceans to absorb C0 2 , the variable atmospheric loading of 
wind-blown dust, and the interaction of C0 2 with the biosphere. 

The atmospheric C0 2 concentrations recorded at Mauna Loa, Hawaii 
(and other locations) show a steady increase in the annual average, 
amounting to about a 4 percent rise in total C0 2 between 1958 and 1972 
(Keeling et ai, 1974). The present-day C0 2 excess (relative to the year 
1850) is estimated at 13 percent. A comparison with estimates of the 
fossil C0 2 input to the atmosphere from human activities indicates that 
between 50 and 75 percent of the latter has stayed in the atmosphere, 
with the remainder entering the ocean and the biosphere. The C0 2 
excess is conservatively projected to increase to 15 percent by 1980, to 
22 percent by 1990, and to 32 percent by 2000 a.d. The corresponding 
changes of mean atmospheric temperature due to C0 2 [as calculated 
by Manabe (1971) on the assumption of constant relative humidity 
and fixed cloudiness] are about 0.3 °C per 10 percent change of C0 2 
and appear capable of accounting for only a fraction of the observed 
warming of the earth between 1880 and 1940. They could, however, 
conceivably aggregate to a further warming of about 0.5 °C between 
now and the end of the century. 

The total global atmospheric loading by small particles (those less 
than 5 ^m in diameter) is less well monitored than is C0 2 content but 
is estimated to be at present about 4xl0 7 tons, of which perhaps as 
much as 1 x 10 7 tons is derived both directly and indirectly from human 
activities. If the anthropogenic fraction should grow in the future at the 
not unrealistic rate of 4 percent per year, the total particulate loading 



44 UNDERSTANDING CLIMATIC CHANGE 

of the atmosphere could increase about 60 percent above its present- 
day level by the end of this century. The present-day anthropogenic 
particulate loading is estimated to exceed the average stratospheric 
loading by volcanic dust during the past 120 years but to equal only 
perhaps one fifth of the stratospheric loading that followed the 1883 
eruption of Krakatoa. 

The impact of such particle loading on the mean atmospheric tem- 
perature cannot be reliably determined from present information. 
Recent studies indicate that the role of atmospheric aerosols in the heat 
budget depends critically on the aerosols' absorptivity, as well as on 
their scattering properties and vertical distribution. The net thermal 
impact of aerosols on the lower atmosphere (below cloud level) prob- 
ably depends on the evaporable water content of the surface in addition 
to the surface albedo. Aerosols may also affect the structure and 
distribution of clouds and thereby produce effects that are more im- 
portant than their direct radiative interaction (Hobbs et al., 1914; 
Mitchell, 1974). 

Of the two forms of pollution, the carbon dioxide increase is probably 
the more influential at the present time in changing temperatures near 
the earth's surface (Mitchell, 1973a). If both the C0 2 and particulate 
inputs to the atmosphere grow at equal rates in the future, the widely 
differing atmospheric residence times of the two pollutants means that 
the particulate effect will grow in importance relative to that of C0 2 . 

Thermal Pollution, Clouds, and Surface Changes 

There are other possible impacts of human activities that should be 
considered in projecting future climates. One of these is the thermal 
pollution resulting from man's increasing use of energy and the inevitable 
discharge of waste heat into either the atmosphere or the ocean. Al- 
though it is not yet significant on the global scale, the projections of 
Budyko (1969) and others indicate that this heat source may become 
an appreciable fraction (1 percent or more) of the effective solar 
radiation absorbed at the earth's surface by the middle of the next 
century. And if future energy generation is concentrated into large 
nuclear power parks, the natural heat balance over considerable areas 
may be upset long before that time. Recent estimates by Haefele (1974) 
indicate that by early in the next century, the total energy use over the 
continents will approach 1 percent of the natural heat density of about 
50 W/m 2 and that in local industrial areas the man-made energy 
density may become several hundred times larger. 

There is also the possibility that widespread artificial creation of 



PAST CLIMATIC VARIATIONS— PROJECTION OF FUTURE CLIMATES 45 

clouds by aircraft exhaust and by other means may induce significant 
climatic variations, although there is no firm evidence that this has yet 
occurred. Such effects could serve to increase the already prominent role 
played by (natural) clouds in the earth's heat balance (see Figure 3.2). 
Widespread changes of surface land character resulting from agri- 
cultural use and urbanization, and the introduction of man-made sources 
of evaporable water, may also have significant impacts on future 
climates. When the surface albedo and surface roughness are changed 
by the removal of vegetation, for example, the regional climatic 
anomalies introduced may have large-scale effects, depending on the 
location and scale of the changes. The creation of large lakes and 
reservoirs by the diversion of natural watercourses may also have 
widespread climatic consequences. The list of man's possible future 
alterations of the earth's surface can be considerably lengthened by the 
inclusion of more ambitious schemes, such as the removal of ice cover 
in the polar regions and the diversion of ocean currents. Again, how- 
ever, it is only through the use of adequately calibrated numerical 
models that we can hope to acquire the information necessary for a 
quantitative assessment of the climatic impacts. 



5 



SCOPE OF PRESENT RESEARCH 
ON CLIMATIC VARIATION 



The overview of the problem of climatic variation presented in the 
preceding chapters and in the technical appendixes contains only those 
references to the literature that were helpful in the illustration of a 
particular viewpoint or necessary to document a specific source of in- 
formation. In the course of its deliberations, however, the Panel found 
it necessary to survey present research on climatic variation, as rep- 
resented by the more recently published literature and by selected on- 
going activities. Inasmuch as this information may serve as a useful 
background to the Panel's recommendations for a national and inter- 
national program of climatic research, it is summarized here. Even this 
survey, in which emphasis is given to material published since 1970, 
must be considered incomplete and necessarily gives precedence to 
sources of information most readily available to the Panel. Further use- 
ful references on various aspects of the problem of climatic variation 
are to be found in other recent publications (Committee on Polar 
Research, 1970; National Science Board, 1972; Wilson, 1970, 1971). 

CLIMATIC DATA COLLECTION AND ANALYSIS 

Here the current status of efforts to assemble climatic data for both the 
atmosphere and ocean is summarized, and the various observational 
field programs directed to the collection of specific data of climatic 
interest are described. 

46 



SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 



Atmospheric Observations 



47 



Climatological data banks are maintained by noaa's National Climatic 
Center (ncc) and National Meteorological Center (nmc) and by the 
military operational weather services, particularly the Air Force's En- 
vironmental Technical Applications Center (etac) and the Navy's 
Fleet Numerical Weather Central (fnwc). Using data from these 
sources, atmospheric data sets specifically for climatic studies have been 
assembled by the National Center for Atmospheric Research, the Geo- 
physical Fluid Dynamics Laboratory, MIT, and other institutions. 
Efforts to assemble the rapidly accumulating data from meteorological 
satellites have also been made by noaa's National Environmental Satel- 
lite Service (ness) and by the University of Wisconsin. Sustained efforts 
to assemble and systematically analyze such data for the use of the 
climatic research community are important tasks for the future. 

In addition to the standard compilations of climatological statistics 
prepared on a routine basis by governmental agencies, new summaries 
of upper-air data have been prepared (Crutcher and Meserve, 1970; 
Taljaard et al., 1969); these have permitted the initial construction of 
the average monthly global distributions of the basic meteorological 
variables of pressure, temperature, and dew points at selected levels. 
The analysis of such data in terms of the various statistics of the global 
circulation is less advanced, although intensive studies of a five-year 
period in the northern hemisphere have recently been completed (Oort, 
1972; Oort and Rasmusson, 1971; Starr and Oort, 1973). These studies 
provide the most quantitative analyses of the annual climatic variations 
of the atmosphere yet made, and plans are under way for their extension 
to additional five-year periods. 

Studies of the spatial patterns of observed variability over longer time 
periods are almost entirely confined to surface variables in the northern 
hemisphere (Hellerman, 1967; Kutzbach, 1970; Wagner, 1971). Such 
studies should be extended to other portions of the atmosphere and 
broadened to include other, less comprehensively observed climatic 
elements. 

An observational analysis of the tropical and equatorial circulation 
has been completed (Newell et al., 1972), and statistics for the strato- 
spheric climate are becoming increasingly available (Newell, 1972). 
Comprehensive data on the components of the global atmospheric 
energy balance are only beginning to be available (Newell et al., 1969), 
although many rely on older and indirect data for the unobserved ele- 
ments of the heat balance at the earth's surface (Budyko, 1956, 1963; 
Lvovitch and Ovtchinnikov, 1964; Moller, 1951; Posey and Clapp, 



48 UNDERSTANDING CLIMATIC CHANGE 

1964). More recent direct observations from satellites, however, are 
providing valuable new insight into both the time and space variations 
of the overall radiation budget of the earth (Vonder Haar and Suomi, 
1971) and promise to provide further data of climatic importance as 
newer and more versatile satellite observational capabilities develop 
(Chahine, 1974; cospar Working Group 6, 1972; Raschke et al, 1973; 
Smithed/., 1973). 

Oceanic and Other Observations 

The observational data base for the oceans is much less developed than 
that for the atmosphere, and oceanic climatic summaries are based 
largely on observations that are more widely scattered in both space and 
time. Even for the more traveled parts of the oceans, these data are 
sufficient only to indicate the average large-scale features of the ocean's 
structure and circulation (Fuglister, 1960; Hellerman, 1967; Sverdrup 
et al, 1942; U.S. Navy Hydrographic Office, 1944). Updated com- 
pilations of surface stress (Hellerman, 1967)) and sea-surface tem- 
peratures (Alexander and Mobley, 1973; Washington and Thiel, 
1970) have been made, and summaries of the observed subsurface 
temperature structure have recently become available for selected 
oceans (Born et al, 1973; Robinson and Bauer, 1971). 

Significant repositories of oceanic data useful for climatic purposes 
exist at a number of institutions, although a comprehensive oceanic 
data inventory has not yet been prepared. The Navy's Fleet Numerical 
Weather Central, the Scripps Institution of Oceanography, the Woods 
Hole Oceanographic Institution, and the National Marine Fisheries 
Service, for example, all have specialized oceanographic data banks, as 
well as data from individual cruises and expeditions. Guides to the 
oceanic data services of the Environmental Data Service (1973) of 
noaa are also available. 

An increasing amount of data on oceanic surface conditions is becom- 
ing available from satellite observations and other remote-sensing 
techniques (Munk and Woods, 1973; Shenk and Salomonson, 1972) 
and offer the promise of routine global monitoring of sea-surface tem- 
perature and sea-ice distribution. Satellite data collected by ness also 
permit the determination of the snow and ice extent over land; this and 
other glaciological data are being accumulated by the U.S. Geological 
Survey. The further extension of oceanographic, sea-ice, and glaci- 
ological observations by satellites, buoys, and ships is under active 
consideration in connection with the fgge (garp, 1972; Stommel, 
1973) and is part of other large-scale programs as well (International 



SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 49 

Decade of Ocean Exploration, 1973; International Glaciological Pro- 
gramme for the Antarctic Peninsula, 1973; Kasser, 1973; Mid-ocean 
Dynamics Experiment-one, Scientific Council, 1973; Joint U.S. polex 
Panel, 1974). 



Observational Field Programs 

Many observational data of importance to climatic research have been 
acquired in special field programs. Some of these are directly related to 
garp itself (amtex Study Group, 1973; garp Joint Organizing Com- 
mittee, 1972, 1973; Houghton, 1974; Kondratyev, 1973), such as the 
Complete Atmospheric Energetics Experiment (caenex), the Air-Mass 
Transformation Experiment (amtex), the garp Atlantic Tropical Ex- 
periment (gate), and the Arctic Ice Dynamics Joint Experiment 
(aidjex). Others are part of the nsf's International Decade of Ocean 
Exploration (idoe) (1973) program (Mid-ocean Dynamics Experi- 
ment-one, Scientific Council, 1973), such as the Geochemical Ocean 
Sections Study (geosecs), the Mid-ocean Dynamics Experiment 
(mode), the North Pacific Experiment (norpax), and the Climate, 
Long-range Investigation, Mapping, and Prediction (climap) project. 
Other field programs are aimed at the monitoring of atmospheric com- 
position and aerosols, such as those of ncar, the Environmental Pro- 
tection Agency, and noaa's Environmental Research Laboratories. 

Each of these programs is focused on physical processes of im- 
portance in particular geographical regions and is a valuable source of 
experience and information. There are also international programs of 
this sort in various stages of planning, such as the Polar Experiment 
(polex) (Joint U.S. polex Panel, 1974), the International Glaci- 
ological Program for the Antarctic Peninsula (igpap) (1973), and the 
International Southern Ocean Studies (isos) programs (isos Planning 
Committee, 1973). Cooperative programs such as these will be neces- 
sary for the comprehensive future monitoring, analysis, and modeling of 
climate and climatic variation. 



STUDIES OF CLIMATE FROM HISTORICAL SOURCES 

The record of past climates as contained in various historical documents, 
writings, and archeological material has been increasingly recognized as 
an important source of information (Bryson and Julian, 1963; Butzer, 
1971; Carpenter, 1965; LeRoy Ladurie, 1971; Lamb, 1968, 1972; 
Ludlam, 1966, 1968). These sources permit the study of historical 
climates over the past several thousand years. A systematic compilation 






50 UNDERSTANDING CLIMATIC CHANGE 

of material of this sort is being undertaken by the Climatic Research 
Unit of the University of East Anglia (Lamb, 1973b). 



STUDIES OF CLIMATE FROM PROXY SOURCES 

The assembly of paleoclimatic information from proxy data sources has 
attained new importance in recent years with the development of new 
methods of dating and of new techniques of quantitative climatic in- 
ference. In the following, the various efforts in this aspect of climatic 
research are briefly summarized. 



General Syntheses 

Two broad surveys of paleoclimatology have appeared in recent years 
(Funnell and Riedel, 1971; Schwartzbach, 1961), along with textbooks 
(Flint, 1971; Washburn, 1973) and symposia (Black et al, 1973; 
Turekian, 1971), which emphasize the glacial processes during the late 
Cenozoic period. Other recent paleoclimatological syntheses have been 
concerned with the broad range of Quaternary studies (Wright and 
Frey, 1965), with the relationships between Pleistocene geology and 
biology (Butzer, 1971; West, 1968), and with more recent paleoclimatic 
fluctuations from a meteorological viewpoint (Lamb, 1969). The 
review of the full range of paleoclimatic events on all time scales given 
in Appendix A of this report has been made possible by the recent 
application of improved dating methods to the stratigraphic record, of 
ocean sediments and uplifted reefs. This synthesis illustrates the essential 
need for an accurate time scale in the interpretation of proxy climatic 
data. 



Chronology 

The methods of dendrochronology (Ferguson, 1970; LaMarche and 
Harlan, 1973), the radiocarbon method (Olsson, 1970; Wendland and 
Bryson, 1974), and other isotopic dating methods have recently been 
used to infer the chronology of climate over the past several hundred 
thousand years (Broecker and van Donk, 1970; Matthews, 1973; 
Mesolella et al, 1969). Biostratigraphic and paleomagnetic correlations 
between the marine and continental records have provided a reasonably 
accurate chronology of the past 60 million years by the use of potassium- 
argon and other isotopes (Berggren, 1971, 1972; Hays et al, 1969; 
Kukla, 1970; Ruddiman, 1971; Sancetta et al, 1973; Shackleton and 
Kennett, 1974a, 1974b; Shackleton and Opdyke, 1973). 



SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 51 

Monitoring Techniques 

Following the initial efforts to estimate paleotemperatures from isotopic 
time series (Emiliani, 1955, 1968), recent work has made it possible 
to separate the effects of temperature from those of ice-volume change 
(Shackleton and Opdyke, 1973). Multivariate statistical techniques 
have recently been developed that permit the quantitative estimation of 
climatic parameters from the concentration of fossil plankton in deep- 
sea sediments (Imbrie and Kipp, 1971; Imbrie et al, 1973; Kipp, 
1974), the growth record of tree rings (Fritts et al, 1971), and the 
continental distribution of fossil pollen (Webb and Bryson, 1972). 
These methods have since been applied to the reconstruction of paleo- 
ocean temperatures (Luz, 1973; Mclntyre et ah, 1972a; Pisias et al., 
1973; Sachs, 1973), as well as to pressure and precipitation anomalies 
(Fritts, 1972). Isotopic studies of cores taken in the polar ice caps 
provide measures of the air temperature at the time of ice formation 
(Dansgaard et al., 1971). Further refinements of such monitoring tech- 
niques will help to fill in the paleoclimatic record, especially when 
several independent methods are available for the same period. 

Proxy Data Records and Their Climatic Inferences 

Proxy data come from a wide variety of sources; potentially, any bio- 
logical, chemical, or physical characteristic that responds to climate 
may provide proxy data useful in the reconstruction of past climates. 
One of the more prolific sources of long-term climatic information has 
been the extensive collection of deep-sea cores, obtained routinely over 
the years on various oceanographic expeditions and more recently 
from the Deep-Sea Drilling Project (Douglas and Savin, 1973; Shackle- 
ton and Kennett: 1974a, 1974b). Analysis of the fossil flora and fauna 
in such cores, with chronology provided from their isotopic content 
and paleomagnetic stratigraphy, has been performed for all the princi- 
pal oceans of the world (Emiliani, 1968; Gardner and Hays, 1974; 
Hunkins et al, 1971; Imbrie, et al, 1973; Kellogg, 1974; Kennett and 
Huddlestun, 1972; Moore, 1973) and provides a preliminary docu- 
mentation of the temperature and large-scale displacements of the 
surface waters during the last few hundred thousand years (Mclntyre 
et al, 1972b; Shackleton and Opdyke, 1973). Other characteristics of 
the sediment cores, such as the presence of volcanic ash (Ruddiman 
and Glover, 1972), also indicate climatically important events, as well 
as providing valuable core dating horizons. For periods of particular 
interest, such as the glacial maximum of about 18,000 years ago, de- 



52 UNDERSTANDING CLIMATIC CHANGE 

tailed reconstructions of seasonal sea-surface temperature and salinity 
have been made for the North Atlantic (Mclntyre et al, 1974) and 
more recently have been extended to the world ocean under the 
climap program. 

The concentration of fossil pollen and the record of soil types in 
relatively undisturbed continental sites is another source of proxy 
data on terrestrial paleoclimates. In recent years, pollen data have 
been analyzed from a number of continental areas (Bernabo et al, 1974; 
Davis, 1969; Heusser, 1966; Heusser and Florer, 1974; Livingstone, 
1971; Swain, 1973; Tsukada, 1968; van der Hammen et al, 1971) 
and provide a preliminary documentation of the surface vegetational 
changes during the late Cenozoic and Quaternary periods (Leopold, 
1969; Wright, 1971). Soil records have been studied less extensively 
but provide corroborative evidence of surface climatic conditions (Frye 
and Willman, 1973; Kukla, 1970; Sorenson and Knox, 1973). 

In many ways analogous to the records from deep-sea cores, proxy 
climatic data from ice cores have recently been obtained from sites in 
both Antarctica and Greenland (Dansgaard et al, 1969, 1971). Such 
ice-core records provide a detailed history of atmospheric conditions 
over the ice during the last hundred thousand years (Dansgaard et ah, 
1973; Johnsen et al, 1972; Langway, 1970). The drilling of deeper 
cores are planned, and their analysis and correlation with other proxy 
data will contribute significantly to the reconstruction of global climatic 
history. 

Further climatic inferences are obtained from proxy data on marine 
shorelines. By assembling data on dated terraces at selected continental 
and island sites, and with the necessary adjustments for eustatic changes 
in the earth's crust, the record of sea-level variations over the last 
150,000 years is becoming established (Bloom, 1971; Currey, 1965; 
Matthews, 1973; Mesolella et al, 1969; Milliman and Emery, 1968; 
Steinen et al, 1973; Walcott, 1972), particularly as regards the timing 
of high stands. 

Closely related to the questions of ice, soil, and sea-level changes 
are the proxy data from glacial fluctuations themselves. Considerable 
attention has been given in recent years to the reconstruction of the 
glacial history of the most recent major ice age in North America 
(Andrews et al, 1972; Black et al, 1973; Dreimanis and Karrow, 1972; 
Frye and Willman, 1973; Paterson, 1972; Porter, 1971; Richmond, 
1972), as well as the relatively small but significant fluctuations in 
mountain glaciers over the past 10,000 years (Denton and Karlen, 
1973). Although local glacial margins fluctuate primarily in response 
to the glacier's net mass accumulation, their overall pattern provides 



SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 53 

evidence of larger-scale and longer-period climatic responses. When 
these changes are combined with the more limited data on the glacial 
history of the Antarctic ice sheet, a number of worldwide relationships 
in the major fluctuations of glacial extent begin to emerge (Denton et al., 
1971; Hughes, 1973). 

In the postglacial period, important proxy data on climatic variations 
over the continents also come from the records of tree rings and closed- 
basin lakes. Both of these features respond directly to the hydrologic 
and thermal balances at the surface and when properly calibrated for 
local effects can provide a record of climate over thousands of years. 
With the introduction of new dating and analysis methods, the records 
of tree-ring width variations from both living and fossil trees provide 
an annually integrated record of climatic changes, especially in arid 
regions (Ferguson, 1970; Fritts, 1971, 1972; LaMarche, 1974; La- 
Marche and Harlan, 1973). The radiocarbon dating of debris in selected 
arid lakes provides further evidence of climatic variations, particularly 
as they affect the local water balance (Broecker and Kaufman, 1965; 
ButzeretaL, 1972; Farrand, 1971). 



Institutional Programs 

Much of the present research on paleoclimates is performed in con- 
junction with other glaciological and geological programs, such as those 
of the U.S. Geological Survey, the Illinois Geological Survey, the 
Lamont-Doherty Geological Observatory of Columbia University, and 
the Army's Cold Regions Research and Engineering Laboratory. Other 
efforts are conducted within the larger oceanographic research labora- 
tories, such as the Scripps Institution of Oceanography of the University 
of California, the Woods Hole Oceanographic Institution, the U.S. 
Naval Oceanographic Laboratory, and the marine research laboratories 
of the University of Miami, the University of Rhode Island, and Oregon 
State University. In recent years, more specialized paleoclimatic re- 
search efforts have been developed at a number of other universities, 
joining the long-established Laboratory of Tree-ring Research of the 
University of Arizona and the Institute for Polar Research at The Ohio 
State University. These include the Quaternary Research Centers at 
the University of Washington and the University of Maine, the Center 
for Climatic Research at the University of Wisconsin, the Institute of 
Arctic and Alpine Research at the University of Colorado, and the 
paleoclimatic research- programs in the geology and geophysics depart- 
ments of Brown University and Yale University. 

Notable among the many cooperative activities of these and other 



54 UNDERSTANDING CLIMATIC CHANGE 

institutions are the nsf's idoe programs, including the climap and 
norpax projects. Such cooperative programs have been instrumental 
in developing an effective collaboration among the paleoclimatic, 
oceanographic, and meteorological research communities and should be 
broadened in the future. 



PHYSICAL MECHANISMS OF CLIMATIC CHANGE 

Although the problem of climatic change has been the subject of 
speculation for over a century, recent research has concentrated on 
the study of specific physical processes and on the interactions among 
various components of the climatic physical system. Here the more 
recent of such efforts are briefly surveyed, together with a review of 
associated empirical, diagnostic, and theoretical studies. 



Physical Theories and Feedback Mechanisms 

Of particular interest in the problem of climatic change is the question 
of the cause of the ice ages. Among the recent attempts to answer 
this question are hypotheses that focus upon the roles of sea ice (Donn 
and Ewing, 1968) and ice shelves (Wilson, 1964), the carbon dioxide 
balance (Plass, 1956), and the ocean's salinity (Weye, 1968). Other 
hypotheses emphasize the roles of variations of external boundary con- 
ditions, particularly the incoming solar radiation (Alexander, 1974; 
Budyko, 1969; Clapp, 1970; Manabe and Wetherald, 1967) and the 
volcanic dust loading of the atmosphere (Lamb, 1970) . 

It is generally believed that the astronomical variations of seasonal 
solar radiation play a role in longer-period climatic changes (Milanko- 
vitch, 1930; Mitchell, 1971b; Vernekar, 1972), although there is no 
agreement on the physical mechanisms involved. Recent studies have 
also been made of the long-standing question of possible short-term 
relationships between the climate and solar activity itself (Roberts, 
1973; Roberts and Olson, 1973). Other hypotheses of climatic change 
reckon with the possibility that much of the observed variations of 
climate are essentially the result of the natural, self-excited variability 
of the internal climatic system (Bryson, 1974; Mitchell, 1966, 1971b; 
Sawyer, 1966). 

Of the many feedback processes involved in climate (Schneider and 
Dickinson, 1974) the role of aerosols has recently received particular 
attention (Chylek and Coakley, 1974; Joseph et al, 1973; Mitchell, 
1971a, 1974; Rasool and Schneider, 1971; Schneider, 1971). Although 
our knowledge of the physical properties and global distribution of 



SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 55 

aerosols is limited, these studies indicate that the climatic effects may 
be substantial (Rasool and Schneider, 1971; Yamamoto and Tanaka, 
1972). Several research programs on aerosols are under way, including 
the Global Atmospheric Aerosol Research Study (gaars) of ncar and 
the Soviet caenex program (Kondratyev, 1973) previously noted. 
Attention has also been focused on the regulatory roles of cloudiness 
(Cox, 1971; Mitchell, 1974; Schneider, 1972) and air-sea interaction 
(Namias, 1973; White and Barnett, 1972) in the global climatic sys- 
tem. In both cases, however, an adequate quantitative understanding 
has not yet been achieved. 



Diagnostic and Empirical Studies 

Related to the search for physical climatic theories and mechanisms 
are many empirical and diagnostic studies of various aspects of climatic 
change. Particular attention has been given to the analysis of the large- 
scale variations of the atmospheric circulation that have been observed 
during the past few decades (Angell et al., 1969; Bjerknes, 1969; Davis, 
1972; Namias, 1970; Wahl, 1972; Wahl and Lawson, 1970; White and 
Walker, 1973) and to their relationship to regional anomalies of 
temperature and rainfall (Landsberg, 1973; Namias, 1972b; Winstanley, 
1973a, 1973b). Satellite observations of the large-scale variations of 
surface albedo and seasonal snow cover have brought new attention 
to these features of the climatic system (Kukla and Kukla, 1974; 
Wagner, 1973), as well as necessitating a significant revision of the 
atmospheric radiative energy budget (London and Sasamori, 1971) 
and the estimated oceanic energy transport (Vonder Haar and Oort, 
1973). 

Several recent diagnostic and empirical studies have also focused 
on aspects of the atmosphere-ocean interaction on seasonal, annual, and 
decadal time scales (Lamb and Ratcliffe, 1972; Namias, 1969, 1971b, 
1972a) and have prompted new attention to their relevance to long- 
range forecasting (Ratcliffe, 1973; Ratcliffe and Murray, 1970). The 
larger-scale variations of ocean-surface temperature and sea level have 
also been studied and have led to the identification of apparent tele- 
connections with the atmospheric circulation (Namias, 1971a; Wyrtki, 
1973,1974). 

New studies of mesoscale oceanic features have been made (Bern- 
stein, 1974) and provide further evidence of the dominance of this scale 
in the oceanic energy spectrum (in agreement with the preliminary 
results of the mode program). Other oceanic studies have concentrated 
on the empirical evaluation of the turbulent fluxes of momentum, heat, 



56 UNDERSTANDING CLIMATIC CHANGE 

and water vapor across the air-sea interface (Holland, 1972; Paulson 
et ah, 1971, 1972). The difficulties of estimating the transport of even 
the strongest ocean currents or the heat balance over ice-covered seas 
with the present data base have also received renewed emphasis 
(Fletcher, 1972; Niiler and Richardson, 1973; Reid and Nowlin, 1971). 



Predictability and Related Theoretical Studies 

An important problem in climatic variation is the determination of the 
degree of predictability that is inherent in the natural system, as well 
as that which is achievable by simulation. A number of recent studies 
of simplified models have shown that multiple climatic solutions may 
exist under the same external conditions (Budyko, 1972b; Faegre, 
1972; Lorenz, 1968, 1970) in a manner suggestive of certain features 
of the observed climatic record. There is also evidence from simplified 
models that the completely accurate specification of a climatic state 
is not achievable in any case, because of the same kind of nonlinear 
error growth that limits the accuracy of weather prediction (Fleming, 
1972; Houghton, 1972; Leith, 1971; Lorenz, 1969; Robinson, 1971a). 
Analyses of selected climatic time series indicate only limited pre- 
dictability on yearly and perhaps decadal time scales (Kutzbach and 
Bryson, 1974; Leith, 1973; Lorenz, 1973; Vulis and Monin, 1971), 
while the general white-noise character of higher-frequency fluctuation 
has been confirmed in model simulations (Chervin et al., 1974). Further 
studies of climatic predictability are needed in order to identify both 
the intrinsic and practical limits of climatic prediction. 

NUMERICAL MODELING OF CLIMATE AND CLIMATIC VARIATION 

The accurate portrayal of global climate is the scientific goal of much 
of the atmospheric and oceanic numerical modeling effort now under 
way (Smagorinsky, 1974). When such models are coupled, the direct 
numerical simulation of at least the shorter-period climatic variations 
becomes a realistic possibility. The study of longer-period climatic 
variations, however, may require the construction of increasingly 
parameterized models. Here the more recent modeling research in both 
of these approaches is briefly reviewed. 

Atmospheric General Circulation Models and Related Studies 

Studies with global atmospheric general circulation models (gcm's) 
have focused on the simulation of seasonal climate, with emphasis on 






SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 57 



the analysis of the surface heat and hydrologic balances (Gates, 1972; 
Holloway and Manabe, 1971 ; Kasahara and Washington, 1971 ; Manabe, 
1969a, 1969b; Manabe et al, 1972; Somerville et al, 1974). As de- 
scribed more fully in Appendix B, simulations of average January 
climate have now been achieved by several gcm's. Although additional 
global gcm's are under development (Corby et al, 1972), only two at 
this writing have been integrated over time longer than one year 
(Manabe et al, 1972, 1974b; Mintz et al., 1972). 

Global atmospheric models have also recently been applied to the 
simulation of specific regional circulations, such as those in the tropics 
(Manabe et al., 1974). In such applications the model's parameteriza- 
tion of processes in the surface boundary layer is of particular 
importance (DeardorfT, 1972; Delsol et al., 1971; Sasamori, 1970). Con- 
siderable recent interest has also been shown in the simulation of strato- 
spheric climate with global gcm's (Kasahara and Sasamori, 1974; 
Kasahara et al., 1973; Mahlman and Manabe, 1972). An overview 
of global atmospheric (and oceanic) gcm's is given in Appendix B; 
more detailed reviews of these and other models have recently been 
prepared (Robinson, 1971b; Schneider and Kellogg, 1973), while 
others are in preparation (garp Joint Organizing Committee, 1974; 
Schneider and Dickinson, 1974). 

Statistical-Dynamical Models and Parameterization Studies 

Research on the development of dynamical climate models (in which 
the transfer properties of the large-scale eddies are statistically 
parameterized rather than resolved as in the gcm's) has accelerated in 
recent years (Willson, 1973). These models include those that address 
only the surface heat balance (Budyko, 1969; Faegre, 1972; Sellers, 
1969, 1973), those that consider the time-dependent zonally averaged 
motion (MacCracken, 1972; MacCracken and Luther, 1973; Saltzman 
and Vernekar, 1971, 1972; Wiin-Nielsen, 1972; Williams and Davies, 
1965), and those in which the statistical eddy fluxes are represented in 
terms of the large-scale motions themselves (Dwyer and Petersen, 
1973; Kurihara, 1970, 1973). A key problem in such models is the 
correct parameterization of the heat and momentum transports by the 
large-scale eddies. While a completely adequate formulation has not 
yet been achieved, research is continuing by a variety of methods 
(Clapp, 1970; Gavrilin and Monin, 1970; Green, 1970; Saltzman, 1973; 
Smith, 1973; Stone, 1973). Because of the generally longer time scales 
involved in the oceanic general circulation, relatively less attention has 
been given to the corresponding formulation of statistical-dynamical 



58 UNDERSTANDING CLIMATIC CHANGE 

ocean models (Adem, 1970; Petukhov and Feygel'son, 1973; Pike, 
1972). This problem, however, will assume greater importance with 
the increased development of coupled ocean-atmosphere systems re- 
viewed below. 



Oceanic General Circulation Models 

Although generally less advanced than their atmospheric counterparts 
oceanic gcm's have recently been developed to the point where suc- 
cessful simulations of the seasonal variations of ocean temperature 
and currents have been achieved in both idealized basins (Bryan, 1969; 
Bryan and Cox, 1968; Haney, 1974) and in selected ocean basins with 
realistic lateral geometry (Cox, 1970; Gait, 1973; Holland and Hirsch- 
man, 1972; Huang, 1973). The numerical simulation of the complete 
world ocean circulation has only recently been achieved with baroclinic 
models (Alexander, 1974; Cox, 1974; Takano et al, 1973); this shows 
significant improvement over earlier global simulations with homoge- 
neous wind-driven models (Bryan and Cox, 1972; Crowley, 1968). As 
noted earlier, such models have not yet been able to resolve the energetic 
oceanic mesoscale eddies, although a number of experimental calcula- 
tions to that end are under way. 

Recent studies have also shown the importance of improving the 
models' treatment of the oceanic surface mixed layer (Bathen, 1972; 
Denman, 1973; Denman and Miyake, 1973) and sea ice (Maykut and 
Untersteiner, 1971) and of incorporating bottom topography (Holland, 
1973; Rooth, 1972) and the abyssal water circulation (Kuo and 
Veronis, 1973). 

Coupled General Circulation Models 

Although preliminary numerical calculations with a model of the 
coupled atmosphere-ocean circulation were performed several years ago 
(Manabe and Bryan, 1969; Wetherald and Manabe, 1972), it is only 
recently that a truly globally coupled model has been achieved (Bryan 
et al., 1974; Manabe et al, 1974a). These calculations underscore the 
importance of the ocean's participation in the processes of air-sea 
interaction and in the maintenance of large-scale climate. These and 
other such coupled models now under construction will lay the basis 
for the systematic exploration of the dynamics of the atmosphere-ocean 
system and its role in climatic variation. The necessary calibration and 
testing of coupled gcm's will require a broad data base and access to 
the fastest computers available. 



SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 59 

APPLICATIONS OF CLIMATE MODELS 

The uses of climate models extend across a wide range of applications, 
including the reconstruction of past climates and the projection of 
future climates. Here the more recent use of models for such studies 
is briefly reviewed, as distinguished from the research on model design 
and calibration reviewed above. 

Simulation of Past Climates 

By assembling the boundary conditions appropriate to selected periods 
in the past, numerical models may be applied to the simulation of 
paleoclimates. The climate of the last ice age has recently received 
increased attention, , both through the application of parameterized 
and empirical models (Alyea, 1972; Lamb and Woodroffe, 1970; Mac- 
Cracken, 1968) and through the use of atmospheric gcm's (Kraus, 
1973; Williams et al, 1973). In the latter case, the specification of the 
distribution of glacial ice and sea-surface temperature represents a 
strong thermal control over the simulated climate. In order to provide 
realistic information on the near-equilibrium ice-age climatic state, 
these conditions should be constructed on the basis of the appropriate 
proxy climatic records, while other portions of this same paleoclimatic 
data base serve as verification. An initial effort of this sort is now 
under way as part of the climap program. 

At the present time, the simulation of the time-dependent evolution 
of past climates over thousands of years can only be achieved with the 
more highly parameterized models; the design and calibration of such 
models of the air-sea-ice system are largely tasks for the future. 

Climate Change Experiments and Sensitivity Studies 

Numerical climate models also permit the examination of the climatic 
consequences of a wide variety of possible changes in the physical sys- 
tem and its boundary conditions; such studies, in fact, are a primary 
motivation for the development of the climatic models themselves. As 
previously noted, a number of experiments on the effect of solar radia- 
tion changes have been performed with simplified models (Budyko, 
1969; Manabe and Wetherald, 1967; Schneider and Gal-Chen, 1973; 
Sellers, 1969, 1973), and further studies of this kind with global models 
are under way. A number of recent experiments have been made with 
atmospheric gcm's on the effects of prescribed sea-surface temperature 
anomalies on the large-scale atmospheric circulation (Houghton et al, 



60 UNDERSTANDING CLIMATIC CHANGE 

1973; Rowntree, 1972; Spar, 1973a, 1973b), while others have been 
concerned with the climatic effects of thermal pollution (Washington, 
1972) and of sea ice (Fletcher 1972). 

Although these experiments indicate that the models display a re- 
sponse over several months' time to small changes in the components 
of the surface heat balance, their longer-term climatic response is not 
known. Such experiments serve to emphasize the need for extended 
model integrations, preferably with coupled models, and underscore 
the importance of determining the models' sensitivity and the conse- 
quent noise levels in model-generated climatic statistics. The reduction 
of this climatic noise has an important bearing on the determination of 
the significance of climatic variations (Chervin et al, 1974; Gates, 
1974; Gilman et al, 1963; Leith, 1973). This question is also closely 
related to the problem of long-range or climatic prediction (Brier, 1968; 
Kukla^a/., 1972; Lamb, 1973a). 

Studies of the Mutual Impacts of Climate and Man 

Although the influence of man's activities on the local climate has 
long been recognized, renewed attention has been given in recent years 
to the possibility that man's increasingly extensive alteration of the 
environment may have an impact on the large-scale climate as well 
(Sawyer, 1971). Here the more recent of such studies are briefly re- 
viewed, along with studies of the parallel problem of climate's impact on 
man's activities themselves. 

Aside from the numerical simulations of anthropogenic climatic 
effects noted earlier, there have been a number of recent studies of 
the climatic consequences of atmospheric pollution (Bryson and Wend- 
land, 1970; Mitchell, 1970, 1973a, 1973b; Newell, 1971; Yamamoto 
and Tanaka, 1972) and of the possible effects of man's interference 
with the surface heat balance, primarily through changes of the surface 
land character (Atwater, 1972; Budyko, 1972a; Landsberg, 1970; 
Sawyer, 1971). Aside from local climatic effects, such as those due to 
urbanization, these studies have not yet established the existence of a 
large-scale anthropogenic climatic impact (Machata, 1973). Like their 
numerical simulation counterparts, such studies are made more diffi- 
cult by the high levels of natural climatic variability and by the lack of 
adequate observational data. 

A longer-range question receiving increased attention is the problem 
of disposing of the waste heat that accompanies man's production and 
consumption of energy. When projected into the next century, this 
effect poses potentially serious climatic consequences and may prove 



SCOPE OF PRESENT RESEARCH ON CLIMATIC VARIATION 61 

to be a limiting factor in the determination of acceptable levels of 
energy use (Haefele, 1973; Lovins, 1974). These and other aspects of 
man's impact on the climate have been considered extensively in the 
scep and smic reports (Wilson, 1970, 1971). 

Recent attention has also focused on the effects of climatic varia- 
tions on man's economic and social welfare. From a general awareness 
of these effects (Budyko, 1971; Johnson and Smith, 1965; Maunder, 
1970) research has turned to the representation of climatic anomalies 
in terms of the associated agricultural and commercial impacts (Pittock, 
1972) and to the development of climatic impact indices (Baier, 1973). 
Further studies are necessary in order to develop comprehensive climatic 
impact simulation models, with both diagnostic and predictive capability. 



6 



A NATIONAL CLIMATIC RESEARCH PROGRAM 



While there is ample evidence that past climatic changes have had 
profound effects on man's activities, future changes of climate promise 
to have even greater impacts. The present level of use of land for 
agriculture, the use of water supplies for irrigation and drinking, and 
the use of both airsheds and watersheds for waste disposal is approach- 
ing the limit. A change of climate, even if sustained only for a few 
years' time, could seriously disrupt this use pattern and have far-reach- 
ing consequences to the national economy and well-being. To this vul- 
nerability to natural climatic changes we must add the increasing 
possibility that man's own activities may have significant climatic reper- 
cussions. 

If we are to react rationally to the inevitable climatic changes of 
the future, and if we are ever to predict their future course, whether 
they are natural or man-induced, a far greater understanding of these 
changes is required than we now possess. It is, moreover, important 
that this knowledge be acquired as soon as possible. Although much has 
been accomplished, and further research is under way on many prob- 
lems (as summarized in Chapter 5), the mechanics of the climatic 
system is so complex, and our observations of its behavior so incom- 
plete, that at present we do not know what causes any particular climatic 
change to occur. 

Our response to this state of affairs is the recommendation of an 
integrated research program to contain the observational, analytical, 
and research components necessary to acquire this understanding. 

62 






A NATIONAL CLIMATIC RESEARCH PROGRAM 63 

Heretofore the many pieces of the climatic puzzle have been considered 
in relative isolation from each other, a subdivision that is natural to 
the traditional scientific method. We believe, however, that the time has 
now come to initiate a broad and coordinated attack on the problem of 
climate and climatic change. Such a program should not stifle the de- 
velopment of new and independent lines of attack nor seek to assemble 
all efforts under a single authority. On the contrary, its purpose should 
be to provide a coordinating framework for the necessary research on all 
aspects of this important problem, including the strengthening of those 
efforts already under way as well as the initiation of new efforts. Only 
in this manner can our limited resources be used to maximum benefit 
and a balanced and coherent approach maintained. 

THE APPROACH 

From the summary of recent and current research on climate and 
climatic variation presented in Chapter 5, it is clear that considerable 
effort has been devoted to this problem. It is also clear that much 
remains to be done. As an approach to the research program itself, we 
here attempt to summarize what is now known and to identify those 
elements that now make a greatly expanded effort both feasible and 
desirable. 

What Climatic Events and Processes Can We Now Identify? 

From the analysis of accumulated instrumental climatic data, we can 
identify some of the major characteristics of the climatic changes of 
the past few decades. These include the presence of seasonal and 
annual circulation anomalies over large regions of the earth, together 
with some longer-term trends. More recent satellite observations have 
documented changes in worldwide cloudiness, snow cover, and the 
global radiation balance and have served to emphasize the climatic 
role of the oceans. Although the necessary oceanic measurements have 
not yet been made, satellite observations (together with atmospheric 
data) indicate that the oceans accomplish between one third and one 
half of the total annual meridional heat transport. 

From the analysis of selected paleoclimatic data, it appears that 
ancient climates have been somewhat similar in behavior to the present- 
day climate, although the resolution is poorer. These data also suggest 
the presence of seemingly quasi-periodic climatic fluctuations on time 
scales of order 100,000 years, associated with the earth's major 
glaciations. 



64 UNDERSTANDING CLIMATIC CHANGE 

From the solutions of numerical general circulation models, we 
can identify a number of important physical elements in the mainte- 
nance of global climate. Primary among these is the role played by 
convective motions in the vertical heat flux and by the transfers of heat 
at the ocean surface. Climate models also show that the climate is 
sensitive to the extent of cloudiness and to the surface albedo. Recent 
solutions of global atmospheric models have shown that the accuracy 
of the simulations of cloudiness and precipitation is more difficult to 
establish than the average seasonal distribution of the large-scale 
patterns of pressure, temperature, and wind, which are simulated with 
reasonable accuracy (see Appendix B). This may be due to the pre- 
scription of the sea-surface temperature in the atmospheric models, 
serving to mask errors in the models' heat balance. 

Less experience has been gained with oceanic general circulation 
models, although they are capable of portraying the large-scale thermal 
structure of the oceans and the distribution of the major current sys- 
tems when subject to realistic (atmospheric) surface boundary condi- 
tions. These and other models are just beginning to identify the energetic 
mesoscale eddy, which in some ways appears to be the oceanic counter- 
part of the transient cyclones and anticyclones in the atmosphere. 

From the analysis of a variety of climate models, as well as from 
the analysis of climatic data, we can identify a number of links or 
processes in the phenomenon of climatic change. On at least the shorter 
climatic time scales, the climatic system is regulated by a number of 
feedback mechanisms, especially those involving cloudiness, surface 
temperature, and surface albedo. Underlying these effects is the increas- 
ing evidence that large-scale thermal interactions between the ocean 
and atmosphere are the primary factor in climatic variations on time 
scales from months to millenia. These interactions must be examined 
in coupled ocean-atmosphere models, whose development has just 
begun. The role of the oceans in the climatic system raises the pos- 
sibility of some degree of useful predictability on, say, seasonal or 
annual time scales and is an obviously important matter for further 
research. 

From the analysis of the limited data available, we can identify a 
number of areas in which man's actions may be capable of altering 
the course of climatic change. Chief among these is interference with 
the atmospheric heat balance by increasing the aerosol and particulate 
loading and increasing the C0 2 content of the atmosphere by industrial 
and commercial activity. While present evidence indicates that these 
are not now dominant factors, they may become so in the future. To 
these we must also add the possibility of man's direct thermal inter- 






A NATIONAL CLIMATIC RESEARCH PROGRAM 65 

ference with climate by the disposal of large amounts of waste heat 
into the atmosphere and ocean. Although important large-scale thermal 
pollution effects of this sort do not appear likely before the middle 
of the next century, they may eventually be the factor that limits the 
climatically acceptable level of energy production. 

Why Is a Program Necessary? 

Although the conclusions identified above represent important research 
achievements, they are nevertheless concerned with separate pieces of 
the problem. What we cannot identify at the present time is how the 
complete climatic system operates, which are its most critical and sensi- 
tive parts, which processes are responsible for its changes, and what are 
the most likely future climates. In short, while we know something about 
climate itself, we know very little about climatic change. 

From among the present activities we can identify important prob- 
lems requiring further research. In general, these concern new observa- 
tions and the further analysis of older ones, the design of improved 
climatic models of the atmosphere and ocean, and the simulation of 
climatic variations under a variety of conditions for the past, present, 
and future. As we attempt this research on a global scale, it becomes 
increasingly important that we ensure the smooth flow of data and 
ideas, as well as of resources, among all parts of the problem. The 
attention devoted in each country (and internationally through garp) 
to the improvement of weather forecasting (a problem whose physical 
basis is reasonably well understood) must be matched by a program 
devoted to climate and climatic variation, a problem whose global 
aspects are even more prominent and whose physical basis is not at all 
well understood. The need for a broad, sustained, and coordinated 
attack is therefore a fundamental reason for a climatic research program. 

Other circumstances also indicate that a major research program 
on climatic change is both timely and necessary. First, for the past 
few years we have had available to us the unprecedented observational 
capability of meteorological satellites. This capability has steadily in- 
creased from the initial observations of the cloudiness, radiation budget, 
and albedo to include the vertical distribution of temperature and 
moisture, the extent of snow and ice, the sea-surface temperature, the 
presence of particulates, and the character of the land surface. The 
regular global coverage provided by such satellites clearly constitutes 
an observational breakthrough of great importance for climatic studies. 

Second, the steady increase in the speed and capacity of computers, 
which has been taking place since their introduction in the 1950's, has 



66 UNDERSTANDING CLIMATIC CHANGE 

reached the point where numerical integration of global circulation 
models over many months or even years is now practical. Such calcula- 
tions, along with the associated data processing, will form the quantita- 
tive backbone of climatic research for many years to come, and their 
feasibility clearly constitutes a computational breakthrough. This com- 
puting capability, as represented, for example, by machines of the 
ti-asc or illiac-4 class, will permit extensive experimentation for the 
first time with the coupled global climatic system. 

Finally, the recent development of unified physical models of the 
coupled ocean-atmosphere may itself be viewed as a modeling break- 
through of great importance. Up to now either the atmosphere or the 
ocean has been considered as a separate entity in global modeling, and 
their solutions have consequently described a sort of quasi-equilibrium 
climate. The simulation of climatic variation with these models, on the 
other hand, is just now beginning. A future modeling breakthrough 
of equally great importance will be the successful parameterization of 
the eddy transports of baroclinic disturbances in the atmosphere and 
in the ocean. 

Aside from the practical importance (or even urgency) of the climatic 
problem, the breakthroughs noted above indicate that a time is at 
hand during which progress will be in proportion to our efforts. By co- 
ordinating these efforts into a coherent research program, we may 
therefore expect to achieve significantly greater understanding of 
climatic variation. 



THE RESEARCH PROGRAM (NCRP) 

We have here assembled our specific recommendations for the data, 
the research, and the applications that we believe constitute the needed 
elements of a comprehensive national research program on climatic 
change. We recognize that some of the elements of this program re- 
quire considerable further development and coordination. We also 
recognize that some of the recommended efforts are already under way 
or are planned by various groups, but we believe that their identification 
as parts of a coherent program is both valuable and necessary. Our 
recommendations for the planning and execution of this program are 
given later in this chapter, including those items on which we urge im- 
mediate action. 



Data Needed for Climatic Research 

The availability of suitable climatic data is essential to the success of 
climatic analysis and research, and such data are an integral part of 



A NATIONAL CLIMATIC RESEARCH PROGRAM 67 

the overall program. The needed data are discussed below in terms of a 
subprogram for climatic data assembly and analysis and a subprogram 
for climatic index monitoring. These are the efforts that we believe to 
be necessary to make the store of climatic data more useful to the 
climatic research community and to ensure the systematic collection of 
the needed climatic data in the future. 

Climatic Data Analysis 

Instrumental Data Instrumental observations of the atmosphere ade- 
quate to depict even a decadal climatic variation are available only 
for about the last half century for selected regions of the northern 
hemisphere, and the observational coverage of the oceans is even poorer 
in both space and time (see Appendix A). In order to assess more ac- 
curately the present data base of conventional observations and the 
needed extensions of such data, a number of efforts should be under- 
taken: 

A worldwide inventory of climatic data should be taken to determine 
the amount, nature, and location of past and present instrumental 
observations of the following variables: surface pressure, temperature, 
humidity, wind, rainfall, snowfall, and cloudiness; upper-air tempera- 
ture, pressure-altitude, wind, and humidity; ocean temperature, salinity, 
and current; the location and depth of land ice, sea ice, and snow; the 
surface insolation, ground temperature, ground moisture, and runoff. 
This inventory should identify the length of the observational record, 
the data quality, and the state of its availability. In addition to the 
usual data sources, efforts should be made to locate data from private 
sources, older records, and unpublished climatological summaries. Al- 
though some of these data have been summarized, no overall inventory 
of this type exists. 

Selected portions of these data should be systematically transferred 
to suitable computer storage, in a format permitting easy access and 
screening by variable, time period, and location. These data should 
then be used to compute in a systematic fashion a basic set of climatic 
statistics for as many time periods and for as many regions of the world 
as possible. These should include the means, the variances, and the ex- 
tremes for monthly, seasonal, annual, and decadal periods, for both 
individual stations and for various ensembles of stations up to and in- 
cluding the entire globe. Research should also be devoted to the effects 
of instrumental errors, observational coverage, and analysis procedures 
on climatic statistics. 

Recognizing that these data have large differences in quality, cover- 



68 UNDERSTANDING CLIMATIC CHANGE 

age, and length-of-record and were often collected as by-products of 
other studies, new four-dimensional climatological data-analysis schemes 
should be developed, based on suitable analysis methods or models, 
to synthesize as much of the missing information as possible while 
making maximum use of the available data. Efforts should also be 
devoted to the design of suitable computerized graphical display and 
output. 

Once such syntheses are available, we recommend that suitable 
climatological diagnostic studies be made using dynamical climate 
models to generate systematically the various auxiliary and unobserved 
climatic variables, such as evaporation, sensible heat flux, surface 
wind stress, and the balances of surface heat, moisture, and momentum. 
Such data, of course, would be artificial but may nevertheless be of 
diagnostic use. Insofar as possible, the pertinent statistics of the 
atmospheric and oceanic general circulations and their energy, mo- 
mentum, and heat balances should be determined. 

The results of such analyses should be made available in the form 
of new climatological atlases, supplementing and extending those now 
available for scattered portions of the record and for selected regions 
of the world. The widely used climatological summaries of Sverdrup 
(1942), Moller (1951), and Budyko (1963), for example, are largely 
based on the subjective analyses of older data of uncertain quality. 
Other analyses are more authoritative (Oort and Rasmusson, 1971; 
Newell et al., 1972) but are in need of extension. 

We wish to emphasize the great importance of the potentially un- 
matched coverage of observations from satellites. Those that are of 
climatic value should be systematically cataloged and summarized and 
made available on as timely a basis as possible. These should include 
observations of cloud cover, snow, and ice extent; planetary albedo; and 
the net radiation balance. As remote techniques for measuring the at- 
mosphere's composition, motion, and temperature structure (and the 
surface temperature of land and ocean) are developed, these data 
should be systematically added to the climatological inventory. They 
should also be used in the analysis and model-based diagnostic efforts 
described above and in the climatic index monitoring program out- 
lined below. The presently available summaries of such data (e.g., 
Vonder Haar and Suomi, 1971) have yielded important new results and 
should be continued on an expanded basis. 

Historical Data As noted in Appendix A, a wealth of information 
has been recorded on past variations of weather and climate in historical 
sources such as books, manuscripts, logs, and journals during the past 



A NATIONAL CLIMATIC RESEARCH PROGRAM 69 

several centuries. While much of these data are fragmentary and not 
of a quality comparable with that of instrumental observations, it is 
nevertheless of value. We therefore recommend that 

An organized effort be made to locate, classify, and summarize 
historical climatic information and to identify and exploit new sources. 
From the studies of this sort that have already been made (e.g., Bryson 
and Julian, 1963; LeRoy Ladurie, 1971; Lamb, 1968, 1972), it is clear 
that these efforts should involve historians, archeologists, and geog- 
raphers on an international scale. 

Efforts be made to relate this material to data from other proxy 
sources whenever possible, and efforts made to interpret and focus 
the material in a climatologically meaningful way. 

Proxy Data We recognize the unique value of proxy data for studies 
of climatic change. Such data are obtained from the analysis of tree-ring 
growth patterns, glacier movements, lake and deep-sea sediments, ice 
cores, and studies of soil and periglacial stratigraphy. Data from tree 
rings, annually layered lake sediments, and some ice cores are capable 
of providing information for individual years, while those from other 
sources provide more generalized climatic information on time scales 
of decades, centuries, and millenia. Such data constitute the only source 
of records for the study of the structure and characteristics fluctuations of 
ancient climates. As discussed in Appendix A of this report, some 
of these past climates were quite different from the present regime and 
provide our only documentation of the extreme states of which the 
earth's climatic system is capable. 

Because all proxy climatic data may contain both bias and random 
error components, it is essential that a variety of independent proxy 
records be studied. It is important that coverage be as nearly global 
as possible, since most of the information on climatic variations is 
contained in the spatial patterns of the data fields. While noting that 
some such activity is already in progress, we urge that the assembly 
and analysis of paleoclimatic data be initially focused on four time 
spans (see below). This represents a strategic decision in order to 
make the best use of the available resources. In each area it is im- 
portant that steps be taken to increase greatly the degree of coordina- 
tion and cooperation within the paleoclimatological community, and 
that the cross-checking of overlapping data sets, the development of 
complementary and independent proxy data sources, and the calibration 
against instrumentally observed data be undertaken whenever possible. 

The last 10,000 years. This is the interval within which we may hope 



70 UNDERSTANDING CLIMATIC CHANGE 

to gain insight into the current interglacial period by the systematic 
assembly of a wide variety of proxy climatic data. This is also the 
interval of greatest practical importance for the immediate future. For 
this period particular attention should be given to six techniques: 

Studies of the structural, isotopic, and chemical properties of tree 
rings should be intensified and extended to a global coverage. Since 
forests cover large areas of the globe, it is possible in principle to de- 
velop climatic records over extensive continental areas and to recon- 
struct the spatial patterns of past climate for the past several centuries 
or millenia. The amount of effort depends on the availability of suitable 
trees and on the resolution required in the climatic reconstruction. Data 
on variations of the density of wood from x-ray techniques and on the 
concentrations of trace elements and of stable isotopes of carbon, hy- 
drogen, and oxygen in well-dated rings should also be developed. 
Special efforts should be made to calibrate the few millenia-long tree- 
ring records with information from other suitable proxy data sources, 
such as pollen, varves, and ice cores. 

Studies of pollen records in lakes and bogs should be extended. Most 
pollen analyses to date have concerned bogs created by the retreat of 
the last continental ice sheet. In order to permit synoptic reconstruc- 
tion of the global vegetational record for the past 10,000 years or so, 
pollen analyses with extensive 14 C dating should be extended to the 
nonglaciated areas of the world, particularly to low-latitude regions 
and to the southern hemisphere. 

Studies of the polar ice caps should be expanded. This should include 
additional short ice cores in widely distributed locations, in both 
Greenland and Antarctica, and more detailed isotopic analyses. 

Studies of the major mountain glaciers should be expanded, to ob- 
tain additional information on the various glaciers' advances and re- 
treats, using chronological control where possible. 

Studies of ocean sediments in the few basins of known high deposi- 
tion rates should be greatly expanded. Particularly near the continental 
margins, the synoptic reconstruction of even the decadal variations 
of sea-surface temperatures (and possibly of currents as well) would 
be of great paleoclimatic interest. This effort will involve lithologic, 
faunal, and isotopic analyses of long cores collected specifically for 
this purpose. 

The records from varved sediments in closed basin lakes or land- 
locked seas should be extended. Such data are particularly sensitive to 
the climatic fluctuations in arid regions and would further our knowledge 
of the long-term behavior of deserts and drought. 

The last 30,000 years. This interval is dominated by the waxing and 






A NATIONAL CLIMATIC RESEARCH PROGRAM 71 

waning of continental ice sheets. In this interval the radiocarbon dating 
method provides a good chronology, and the possibilities for studying 
the relative phases of different proxy climatic records on a global basis 
are a maximum. In this period particular efforts should be made in the 
following areas : 

Pollen records for the interval 10,000 to 30,000 years ago should 
be obtained in a wide variety of sites in both hemispheres. 

Ice-margin data should continue to be collected for northern hem- 
isphere glaciers and should be extended into southern hemisphere 
mountain areas. 

Additional deep-sea cores should be obtained, especially in the 
Pacific and Southern Oceans, in order to reveal further the geographic 
pattern of marine paleoclimates. These data would be particularly useful 
from high-deposition-rate basins. 

Additional data should be obtained on the fluctuations in the extent 
and volume of the polar ice sheets during this time interval. Particular 
attention should be given to the smaller ice sheets, such as the West 
Antarctic and Greenland ice sheets, which react more rapidly to climatic 
variations. 

More extensive analyses of sea-level records should be made, empha- 
sizing the removal of tectonic and isostatic effects. Present studies on 
raised coral reefs should be extended, and estuarine borings should be 
carefully dated and given thorough lithologic analysis. 

The last 150,000 years. Here we should seek to increase our knowl- 
edge of the last 100,000-year glacial-interglacial cycle. This interval 
includes the last period in the climatic history of the earth that was evi- 
dently most like that of today. The data of this period also provide the 
best example of how the last interglacial period ended. Efforts should be 
made to further develop a number of proxy data sources, including: 

Extensive collection and analyses of marine sediment cores to pro- 
vide adequate global coverage of the world ocean. 

Further studies of the fluctuations of the Antarctic and Greenland 
ice caps, with emphasis on records extending beyond the beginning of 
the last interglacial. This should include a geographic network of ice 
cores of sufficient length to penetrate this time range, of which those 
at Camp Century, Byrd, and Vostok are now the only examples. 

Further systematic study of the loess-soil sequences in suitable regions 
around the world, including Argentina, Australia, China, and the Great 
Plains of North America. 

Systematic studies of desert regions and arid intermountain basin 



72 UNDERSTANDING CLIMATIC CHANGE 

areas in order to examine the patterns of long-term changes in aridity. 
Present records are limited to about the last 40,000 years, and their 
extension will require long borings in selected lakes and playas. 

Extended studies of sea-level variations from coral reef and island 
shorelines features. 

Further studies of long pollen records covering previous interglacial 
periods. This should include data from previously unsampled regions 
of the world, particularly in the southern hemisphere. 

The last 1,000,000 years and beyond. Fluctuations in this time range 
should not be ignored simply because of their antiquity. Here we have 
the opportunity to compare the circulation patterns that have char- 
acterized the last several full-glacial and interglacial periods, and 
thereby to contribute evidence on the question of the degree of de- 
terminism of the earth's climatic system. Efforts should therefore be 
made to extend suitable proxy records into this time range, including: 

Additional marine sediment cores of sufficient length (say, up to 
100 m long) to cover several glacial cycles should be obtained. This 
will require new innovations in drilling technology, as piston cores 
do not penetrate deeply enough for this purpose, and rotary drills 
presently in use greatly disturb the sedimentary record. 

The record of the Antarctic ice sheet (and the associated sea-level 
variations) should be extended as far back as possible and in as much 
detail as possible. This ice mass is a living climatic fossil and may 
contain information about the global climate for the past several million 
years. 

Climatic Index Identification and Monitoring In addition to the data 
provided by conventional surface and upper-air observations, climatic 
studies require other contemporary data that are not now readily 
available. The one hope for obtaining truly global coverage of many 
current climatic variables rests with satellite observations. We expect 
that climatic studies in the foreseeable future will have to rely on a 
combination of conventional observations, satellite observations, and 
special observations designed to monitor selected climatic variables 
as discussed below. We should therefore make full use of the temporary 
expansion of the observational network planned for the fgge in 1978 
in order to design a longer-lived climatic observing program. In addition, 
efforts should be made to process the monitored data from both satel- 
lites and other systems into forms that are useful for climatic studies. 
Support should be given to the development of new satellite-based ob- 



A NATIONAL CLIMATIC RESEARCH PROGRAM 73 

servational techniques, including those designed to monitor the oceans 
and the earth's surface. 

There remain, however, a number of processes that are important to 
climate that are now beyond the reach of satellite observations. Primary 
among these is the pattern of the planetary thermal forcing, which 
drives the atmospheric and oceanic circulation, and the related balance 
of energy at the earth's surface. Even a measurement of the average 
pole-to-equator temperature difference tells us something about the 
circulation; and, in a similar way, the discharge of a river gives us 
some information on the hydrologic balance in the river's basin. 

Such measurements, which represent time and space integrals of 
climatically important procesess, we term "climatic indices." While 
efforts to monitor indices of this sort are already under way, we recom- 
mend that further efforts be made to identify and monitor a variety of 
such indices in a coordinated and sustained fashion, as part of a compre- 
hensive global Climatic Index Monitoring Program (cimp) whose 
elements are outlined below. 

Atmospheric Indices The heat balance of the atmosphere is basic 
to the character of the general circulation and hence is a principal de- 
terminant of climate. It is therefore important that the primary ele- 
ments of this balance be monitored with as much accuracy and with 
as nearly global coverage as possible. In particular, we recommend that 
further efforts be made to 

Monitor the solar constant and the spectral distribution of solar 
radiation with appropriate satelliteborne instrumentation. 

Monitor the net outgoing shortwave and long-wave radiation by 
satellite-based measurements, from which determinations of the ab- 
sorbed radiation and planetary albedo may be made. 

Monitor the latent heat released in large-scale tropical convection, 
possibly with the aid of satellite cloud observations. 

Develop methods to monitor remotely the surface latent heat flux 
into the atmosphere, possibly with the aid of satellite measurements 
of the vertical distribution and total amount of water vapor. These 
methods (and those for the sensible heat flux discussed below) will 
require calibration against field appropriate measurements, especially 
over the oceans. 

Develop methods to monitor remotely the surface sensible heat flux 
into the atmosphere, especially that from the oceans, such as occurs in 
winter off the east coasts of the continents and in the higher latitudes. 
Efforts should also be made to monitor remotely the vertical sensible 



74 UNDERSTANDING CLIMATIC CHANGE 

heat flux that occurs as a result of convective motions both over the 
oceans and over land. 

Expand the satellite monitoring of global cloud cover to include 
information on the clouds' height, thickness, and liquid water content, 
so that their role in the heat balance may be determined. 

Monitor the distribution of surface wind over the oceans, possibly 
by radar measurements of the scattering by surface waves or from 
the microwave emissivity changes created by foam. 

Oceanic Indices In view of the fundamental role the oceans play 
in the processes of climatic change, special efforts should be made to 
monitor those oceanic variables associated with large-scale thermal 
interaction with the atmosphere. In addition to the low-level air tem- 
perature, moisture, cloudiness, surface wind, and surface radiation, the 
surface heat exchange depends critically on the sea-surface temperature 
and heat storage in the oceanic surface layer itself. We therefore recom- 
mend that further efforts be made to 

Monitor the worldwide distribution of sea-surface temperature by 
a combination of all available ship, buoy, coastal, and satellite-based 
measurements. Sea-surface temperature analyses, such as now per- 
formed operationally by the Navy's Fleet Numerical Weather Central 
in Monterey, should be extended and supplemented for climatic pur- 
poses on a global basis by improved satellite observations capable of 
penetrating cloud layers. The drifting buoy observations of sea-surface 
temperature planned for the fgge should be expanded and maintained 
on a routine basis. 

Monitor the heat storage in the surface layer of the ocean by a 
program of observations from satellite-interrogated expandable drifting 
buoys and by expendable bathythermograph (xbt) observations from 
ships-of-opportunity in those areas of the world ocean traveled by 
commercial ships. It is estimated that there are several hundred such 
transits each year across most major oceans of the world. An expan- 
sion of xbt observations from merchant ships-of-opportunity is being 
undertaken by the North Pacific project (norpax), in cooperation with 
the Navy's Fleet Numerical Weather Central and noaa's National 
Marine Fisheries Service. Similar programs should be undertaken in 
the other oceans, and especially in the oceans of the southern hemisphere, 
with special efforts made to place instruments aboard ships on uncon- 
ventional routes and on selected government vessels. This xbt program 
should be supplemented by buoy measurements in selected locations 



A NATIONAL CLIMATIC RESEARCH PROGRAM 75 

and by xbt's launched from aircraft on meridional flight paths in the 
more inaccessible ocean areas. 

Expand the present data buoy programs now under way by noaa and 
others, so that the volume and heat transport of the major ocean cur- 
rents can be monitored. Suitably deployed bottom-mounted sensors, 
moored buoys, or both should be used to monitor the transport of the 
Gulf Stream, Kuroshio, and Antarctic circumpolar currents in selected 
locations, such as is planned for the Drake Passage as part of the Inter- 
national Southern Ocean Studies (isos). The water mass balance of 
individual basins such as the Arctic should also be monitored. 

Monitor the complete temperature structure in selected regions of 
the ocean, such as meridional cross sections through the major gyral 
circulations. The several long-term local observational series (such 
as the Panulirus, Plymouth, and Murmansk sections) should be main- 
tained and new efforts started in regions of special interest. 

Monitor the vertical salinity structure of the oceans in those high- 
latitude regions where salinity plays an important role in determining 
the density field of the upper ocean layers. Near-surface salinity is 
also important in regions where ocean bottom water is formed, such 
as in the Weddell Sea. This might best be done by a combination of 
moored buoys and ship observations. 

Monitor the large-scale distribution of sea level by the use of an 
expanded network of tide gauges. Such a measurement program at 
island sites in the equatorial Pacific is being undertaken in connection 
with norpax, and other measurements are planned in the Indian Ocean 
as part of the Indian Ocean Experiment (index). Radar altimeters 
such as those proposed for the seasat-a satellite should also be useful 
for this purpose. 

Monitor the oceanic chemical composition at selected sites and in 
selected sections, including the concentrations of dissolved gases and 
trace substances. Such measurements now being performed as part of 
the geosecs program should be expanded and continued. 

Cryospheric Indices In view of the great influence of snow and ice 
cover on the surface energy balance, further efforts should be made to 

Monitor the distribution of sea ice in the polar oceans and the ice 
in major lakes and estuaries. Efforts should also be made to measure 
as many as possible of the ice's physical properties by remote sensing. 

Devote further study to the current mass budgets of the Antarctic 
and Greenland ice caps, from both glaciological field observations and 



76 UNDERSTANDING CLIMATIC CHANGE 

from airborne and satellite measurements. Such observations should 
include changes in ice-edge locations, in the numbers and sizes of ice- 
bergs, and in the ice caps' firnline height. Methods for the remote aerial 
sensing of surface temperature and possibly ice accumulation rate should 
also be further developed. 

Extend the monitoring of the movement and mass budget of se- 
lected mountain glaciers. 

Monitor the extent, depth, and characteristics of worldwide snow 
cover. 

Surface and Hydrologic Indices In association with the monitoring 
of the elements of the surface heat balance, and of the various oceanic 
and cryospheric climatic indices, initially lower priority but neverthe- 
less important efforts should be made to 

Monitor the natural changes of surface vegetative cover, possibly 
by observations from earth resources satellites. 

Monitor the variations of soil moisture and groundwater, possibly 
by satellite-based techniques. 

Monitor the flow and discharge of the major river systems of the 
world. 

Monitor the level and water balance of the major lakes of the world. 

Monitor the total precipitation (especially rainfall over the oceans), 
possibly by satelliteborne radar observations and surface gauges. 

Composition and Turbidity Indices In view of the role that at- 
mospheric constituents and aerosols play in the heat balance of the 
atmosphere, further efforts should be made to 

Monitor the chemical composition of the atmosphere at a number 
of sites throughout the world, with particular reference to the content 
of C0 2 . Measurements such as those at Mauna Loa should be con- 
tinued and extended to additional selected sites. The composition of 
the higher atmosphere should also be periodically determined, especially 
the water vapor in the stratosphere and the ozone concentration in the 
stratosphere and mesosphere. 

Monitor the total aerosol and dust loading of the atmosphere, to- 
gether with determinations of the vertical and horizontal aerosol distri- 
bution, by an extension of such programs as ncar's Global Atmospheric 
Aerosol Study (gaars). In addition to turbidity measurements, the 
aerosol particle-size distribution and optical properties should be de- 
termined when possible. Efforts should also be made to monitor the 



A NATIONAL CLIMATIC RESEARCH PROGRAM 11 

occurrence of large-scale forest fires and volcanic eruptions, together 
with estimates of their particulate loading of the atmosphere. 

Anthropogenic Indices In view of man's increasing interference with 
the environment, further efforts should be made to 

Monitor the addition of waste heat into the atmosphere and ocean. 
Although the present levels of thermal pollution are relatively small on 
a global basis, steadily increasing levels of energy generation pose a 
threat to the stability of at least the local climate and possibly the 
larger-scale climate as well. Therefore both the local thermal discharges 
of power generating and industrial facilities should be monitored, along 
with the thermal pollution from urbanized areas. 

Monitor the climate-sensitive chemical pollution of the atmosphere 
and ocean. Measurement programs such as those of the Environmental 
Protection Agency and the Atomic Energy Commission should be ex- 
panded on a global basis and extended to the oceans. 

Monitor the changes of large-scale land use, including forest clear- 
ing, irrigation, and urbanization, possibly by the use of earth resources 
satellites. 

Summary of Climatic Index Monitoring A summary of the elements 
of the recommended program is given in Table 6.1. Here we have not 
made an assessment of the required accuracy of the various monitored 
indices, nor has the capability of presently available instrumentation 
been thoroughly reviewed. Further analysis is also needed to determine 
the characteristic variability of each climatic index. In general, the 
surface heat and hydrologic balances should be monitored with an 
accuracy of a few percent, so that space- and time-averaged climatic 
statistics will have at least a 5 percent accuracy. It is important that this 
monitoring activity be undertaken on a continuing and long-term basis 
for at least two decades in order to assemble a meaningful body of 
data for climatic analyses. As noted below, these efforts should be 
coordinated on an international scale and be a part of an international 
climatic program. 

Research Needed on Climatic Variation 

We here outline the research that we believe needs to be performed, 
in terms of model development, theoretical research, and empirical 
and diagnostic studies. While research in some of these areas is already 
under way as part of garp activities in anticipation of the fgge, these 



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80 UNDERSTANDING CLIMATIC CHANGE 

efforts are the necessary ingredients of the much broader climatic 
research program that we recommend be carried out in the years ahead. 

Theoretical Studies of Climatic Change Mechanisms We recognize 
the importance of theoretical studies in a problem as complex as climatic 
variation and the essential interaction that must take place between 
theory and the complementary observational and numerical modeling 
studies. Our present knowledge of the mechanisms of climatic variation 
is so meager, however, and progress in this area so difficult to anticipate, 
that any recommendations are subject to modification as new avenues 
of attack open up or as old ones prove fruitless. There are, however, 
certain fundamental problems to which further study must be directed: 

The question of the degree of predictability of (natural) climatic 
change must be given further theoretical attention. While the local 
details of weather do not appear to be predictable beyond a few weeks' 
time, the consequences of this fact for climatic variations are not clear. 
In such studies a clear definition of the internal climatic system needs 
to be made, and particular attention must be given to the roles of the 
ocean and ice. This question has an obvious and important bearing on 
our eventual capability to predict climatic variation. 

The related question of the possible intransitivity of climatic states 
needs further study, again with particular attention to the oceans and 
ice. The whole question of climatic variation may be viewed as a 
stability problem for a system containing elements with very different 
time constants, and support should be given to such theoretical ap- 
proaches. 

Theoretical research should be directed to the nature and stability 
of the various climatic feedback mechanisms identified earlier, par- 
ticularly those involving the sea-surface temperature, cloudiness, albedo, 
and land-surface character. 

Further theoretical research should be directed to the general prob- 
lem of the development of statistical-hydrodynamical representations 
of climate and to the parameterization of transient phenomena on a 
variety of time and space scales. 

Additional theoretical studies should also be made of specific climatic 
phenomena, such as drought and the growth of arid regions, ice ages and 
the stability of polar ice cover, and the effects of global pollution from 
natural and artificial sources. 

Atmospheric General Circulation Models The global dynamical 
models of the atmospheric general circulation (or gcm's) that have been 



A NATIONAL CLIMATIC RESEARCH PROGRAM 81 

developed in recent years represent the most sophisticated mathe- 
matical tools ever available for the study of this system and are the 
testing grounds for many of our theoretical ideas. The latest versions 
of these models (see Appendix B) embody much of the physics that 
governs the larger scales of atmospheric behavior, along with physical 
parameterization s of smaller-scale processes. In addition to the simula- 
tion of the free-air temperature, pressure, wind, and humidity distribu- 
tions over the globe with a resolution of several hundred kilometers, 
such atmospheric models provide solutions for the various components 
of the heat and moisture balances, such as the fluxes of shortwave and 
long-wave radiation, sensible heat flux, evaporation, precipitation, sur- 
face runoff, and ground temperature. The surface boundary conditions 
usually assumed are the distributions of sea-surface temperature and 
sea ice, and the assumption of a heat balance over land surfaces. After 
a spin-up period of a month or so during which the temperature comes 
into statistical equilibrium (with the sun's heating and the ocean surface 
temperature), the average global climate simulated by such models 
shows a reasonable resemblance to observation; several examples of 
such simulations are shown in Appendix B. 

In order to improve the fidelity of such global atmospheric models 
for the simulation of the various processes of climatic change, and to 
ensure their increased availability for the conduct of climatic experi- 
ments, efforts should be made to 

Improve the models' treatment of clouds, especially those of the 
nonprecipitating high-altitude cirrus and the low-level stratus type. 
Account should be taken of the liquid-water content of clouds and the 
full interaction of clouds with atmospheric radiative transfer. Attention 
should also be given to the modeling of cloud evaporation and advection. 

Improve the parameterization of turbulent, convective, and mesoscale 
processes by comparing the performance of alternative schemes against 
appropriate observations of the fluxes of heat, moisture, and momentum. 
Particular attention should be given to improved parameterizations of 
the fluxes within the surface boundary layer, to the parameterization of 
cumulus convection, and to the treatment of energy flux by gravity 
waves. 

Improve the treatment of ground cover and land usage in the calcula- 
tion of the surface heat and moisture balances. Particular attention 
should be given to the improvement of the prognostic schemes for 
snow cover, as this may prove of importance in seasonal climatic 
variations. 

Parameterize the role of aerosols in such models, so that the effects 



82 UNDERSTANDING CLIMATIC CHANGE 

of both natural and anthropogenic particulates on the heating rate of 
the atmosphere may be determined. 

Improve the numerical resolution of the solutions by the use of finer 
grids (or the use of graded meshes in regions of special interest) and 
increase the computational efficiency by the development of more 
accurate numerical algorithms and improved solution methods. 

Simulate the annual cycle of atmospheric circulation with models 
using observed forcing functions to obtain the surface fluxes of heat, 
momentum, and water vapor. Such numerical integrations are necessary 
in order to ensure adequate model calibration and to simulate climatic 
statistics for the atmosphere. 

Determine the noise level or sensitivity of the model-simulated climate 
to changes in the initial conditions (including random errors) and to 
changes in the parameterizations of the model. Such studies are neces- 
sary in order to determine the physical significance of numerical climate- 
change experiments made with atmospheric models. 

Oceanic General Circulation Models The oceanic general circulation 
models (or gcm's) are generally at a less advanced stage of develop- 
ment than their atmospheric counterparts and have only recently been 
extended to the global ocean (see Appendix B). With the surface bound- 
ary conditions of specified thermal forcing and wind stress (plus the 
kinematic and insulated wall boundary conditions at the bottom and 
lateral sides of the ocean basin), such models simulate with fair ac- 
curacy the large-scale distributions of ocean temperature and current 
with a resolution of several hundred kilometers. If the density structure 
is specified from observations, a model will spin up from rest in a few 
months' time and show a reasonable correspondence with observed drift 
current patterns at the surface. The simulated transport of the major 
western boundary currents in the models is generally less than that 
indicated by available observations but nevertheless quantitatively 
more accurate than the predictions of previous theories. Areas of 
coastal and equatorial upwelling show the same strong relationship to 
the surface wind-stress pattern in the model as is observed in the real 
ocean. 

The more relevant calculation with respect to climate modeling is 
one in which the density field as well as the velocity field is predicted 
from boundary conditions that determine the vertical flux of momentum, 
heat, and water at the ocean surface. However, this problem involves 
much longer time scales — the spin-up time of a prestratified ocean is 
of the order of two or three decades; but if changes in the abyssal 
thermal structure are to be predicted, then the "turnover" time of the 



A NATIONAL CLIMATIC RESEARCH PROGRAM 83 

ocean is the order of several centuries. Preliminary results (see, for ex- 
ample, Bryan and Cox, 1968) show that such models can successfully 
simulate the gross features of the density structure of the world ocean, 
although more detailed calculations must be made to provide a critical 
test. 

In order to improve the accuracy of ocean models and to lay the 
foundation for their successful coupling with atmospheric models, efforts 
should be made to 

Improve our knowledge of the structure, behavior, and role of 
mesoscale eddies in the ocean. In the atmosphere there is a peak in 
the kinetic energy spectrum observed at wavelengths of a few thousand 
kilometers, whereas in the ocean the peak kinetic energy is in eddies 
that have a radius (or quarter-wavelength) of the order of 10 2 km. 
Thus an ocean circulation model requires about an order of magnitude 
greater horizontal resolution to resolve its most energetic eddies than 
does an atmospheric gcm. Further field studies such as those conducted 
under the Mid-ocean Dynamics Experiment (mode), the North Pacific 
Experiment (norpax), and those planned under the joint Soviet- 
American (polymode) experiment, are needed to determine the transfer 
of heat and momentum by such eddies. Such observational experiments 
should provide the basis for the interpretation of high-resolution nu- 
merical experiments, which are necessary to resolve the details of the 
eddy motions and to establish their role in the oceanic general 
circulation. 

Intensify research on the parameterization of turbulent and meso- 
scale motions both in the surface mixed layer and the deeper ocean 
layers, including thermohaline convection, so that the results of field 
measurements may be usefully incorporated into global ocean circulation 
models. 

Improve the prediction of sea-surface temperature and heat transport 
by the inclusion of the depth and structure of the surface mixed layer 
as a predicted variable in oceanic general circulation models. This 
should include experiments on the numerical forecasting of the oceanic 
surface layer, as driven by observed surface conditions, and the forma- 
tion and behavior of pools of anomalously warm or cold water. 

Simulate the annual cycle of sea-surface temperature and currents 
with models using observed forcing functions to obtain the surface 
fluxes of momentum, heat, and water (precipitation minus evaporation). 
Such numerical integrations must be carried out over several annual 
cycles, in order to ensure adequate model calibration and to simulate 
climatic statistics for the ocean. 



84 UNDERSTANDING CLIMATIC CHANGE 

Subject the ocean models to the same kind of diagnostic testing and 
sensitivity analysis as performed for atmospheric models, in order to 
determine the roles of possible oceanic feedback processes and the 
levels of predictability associated with various oceanic variables. 

Apply high-resolution versions of global oceanic circulation models 
(or regional versions thereof) to the study of the behavior of local in- 
tense currents, such as the eddying motion of western boundary cur- 
rents and the structure of equatorial currents. 

Develop more accurate models of sea ice, which include the effects 
of salinity and the dynamic and thermodynamic factors governing the 
distribution of the polar ice packs. The data base being assembled by 
the Arctic Ice Dynamics Joint Experiment (aidjex) in the Beaufort 
Sea should be useful in the design of models that can predict those 
properties of the polar ice pack that are important in the surface heat 
balance, such as the ice thickness and the occurrence of open-water 
leads. 

Search for new computational algorithms for predicting oceanic 
circulation that will provide the greatest accuracy for the least pos- 
sible cost. At present, the methods used in modeling the ocean are 
similar to those used in the atmospheric gcm's. The presence of lateral 
boundaries and the need to resolve mesoscale motions may make 
alternative numerical methods of particular use in numerical ocean 
models. 

Coupled Global Atmosphere-Ocean Models Tests of climatic change 
extending over one or more years are not adequate unless they are made 
with a model of the coupled ocean-atmosphere system. While the un- 
coupled atmospheric and oceanic gcm's are useful for many purposes, 
the thermal and mechanical coupling between the ocean and atmosphere 
is fundamental to climatic variation. We note that a global ocean model 
may require only a fraction of the computational effort needed by an 
atmospheric gcm of the same resolution but emphasize that care must 
be taken to avoid erroneous drift in the simulated climate due to sys- 
tematic biases in the model or in the oceanic initial state. 

Assuming that coupled models (cgcm's) will incorporate the develop- 
ments and improvements recommended above for the separate at- 
mospheric and oceanic models, emphasis should be given to the follow- 
ing research with cgcm's: 

Investigation of the simulated climatic variability, on seasonal and 
annual time scales, of all climatic variables of the coupled system, 
including the simulated exchange processes at the air-sea interface. 



A NATIONAL CLIMATIC RESEARCH PROGRAM 85 

Of particular importance in the coupled models is the simulation of 
the sea-surface temperature, as this has a key role in the evolution 
of the system. This will require integration over many years of simu- 
lated time in order to generate adequate climatic statistics and to 
examine the models' stability. Particular attention should be given 
to evidence of climatic trends and intransitivity in the numerical solu- 
tions. The statistics of such simulations with cgcm's will also prove 
valuable in the calibration of statistical climate models. 

The sensitivity of the climate simulated by coupled models should be 
systematically examined in experiments extending at least through an 
annual cycle. These studies should include the climatic consequences 
of uncertainties in the simulations' initial state (including random 
errors), in the parameterization of the various physical processes (such 
as convection, cloudiness, boundary-layer fluxes, and mesoscale oceanic 
eddies), and in the computational procedures. Such studies are neces- 
sary in order to establish the characteristic noise levels of the models 
and are of great importance in the use of the models for climate ex- 
periments. 

A program of climate change hypothesis testing should be under- 
taken with coupled models, as soon as their stability and calibration 
are reasonably assured. This should include examination of the various 
feedback mechanisms among components of the climatic system, such 
as ice and snow, cloudiness, sea-surface temperature, albedo, radiation, 
and convection. 

The coupled models should be used in a program of long-range 
integrations with observed initial and boundary conditions, in order 
to assess both their overall fidelity and their usefulness as long-range 
or climatic forecasting tools. 

Although not a research task in itself, special efforts should be 
made to appropriately store, analyze, and display the rather staggering 
amounts of data generated during the integration of cgcm's, so that 
subsequent diagnosis can be performed efficiently. 

Statistical-Dynamical Climate Models Although the coupled numeri- 
cal models of the global circulation offer the most comprehensive and 
detailed solutions available, even with the fastest computers envisaged 
relatively few century-long climatic simulations will be possible, and 
it is likely that none will be performed for periods as long as a millenium. 
Such models will therefore find their greatest use in climatic research 
in the exploration of the character of relatively short-period (say annual 
to decadal) climatic variations and in the calibration of other, less- 
detailed models. We therefore emphasize that statistical-dynamical 



86 UNDERSTANDING CLIMATIC CHANGE 

climate models (defined as those in which the structure and motion 
of the individual large-scale transient disturbances are not resolved in 
detail) will have to be used to simulate the longer-period climatic 
variations. While such models provide less resolution of the details of 
climatic change, they may display less climatic noise than do the global 
circulation models. 

In order to ensure the availability of the hierarchy of models needed 
in a comprehensive research program on climatic change, the following 
research should be carried out: 

Statistical-dynamical models of the coupled time-dependent at- 
mospheric and oceanic circulation should be constructed and calibrated 
that embody suitable time- and space-averaged representations of the 
climatic elements. In their extreme form, such models address the 
steady-state globally averaged quantities, while others, for example, 
consider time-dependent zonally averaged variables. Further efforts 
should be made to represent the climatically important land-sea distri- 
bution in such models and to calibrate them systematically against 
observations as well as against other climatic models. 

Simulation of climatic variation over extended time periods should 
be made by the integration of suitably calibrated time-dependent 
statistical-dynamical models. Depending on the time range, appropriate 
components of the climatic system's atmosphere, hydrosphere, cryo- 
sphere, lithosphere, and biosphere should be introduced, along with 
appropriate variations of the external boundary conditions (see Figures 
3.1 and 3.2). 

Coupled time-dependent models in which the global circulation is 
represented by low-order spatial resolution should also be further de- 
veloped, such as those using a limited number of orthogonal components 
or spectral modes. 

Coupled models should be constructed and calibrated that embody 
new forms of time-averaged representations of the climatic system. We 
recognize that the parameterization of the effects of the transient eddies 
poses a difficult problem in statistical hydrodynamics and urge that 
full use be made of both model-generated and observed statistics, as 
well as of theory, to develop a variety of such models for different types 
and ranges of time averaging. 

In each type of statistical-dynamical model, particular attention 
should be given to the inclusion of the ocean and ice. In such models, 
attention should also be given to the possibility of treating the at- 
mosphere statistically while simulating the ocean in detail and perhaps 
of treating both the atmosphere and ocean statistically while simulating 



A NATIONAL CLIMATIC RESEARCH PROGRAM 87 

the growth of ice sheets in detail. It is particularly important that such 
models be calibrated with respect to both the mean and variance of the 
climatic elements and that their stability and sensitivity be systematically 
determined. 

Empirical and Diagnostic Studies of Climatic Variation Although we 
have recommended some diagnostic and empirical studies in connection 
with the analysis of instrumental and proxy climatic data, such studies 
should also be made on a phenomenological basis as part of the climatic 
analysis and research program. As the record of past climates is made 
more complete, there will be increased opportunity to carry out such 
investigations with both instrumental and proxy data. In particular: 

Studies should be made of the temporal and spatial correlations 
among various data, including regional and global estimates of the 
trends of key climatic elements such as temperature and precipitation. 

Further empirical studies should be made of the surface oceanic 
variables of temperature, salinity, sea level, and sea ice and of the 
planetary heat balance, albedo, and cloudiness from satellite-based ob- 
servations. The studies of Bjerknes (1969), Kukla and Kukla (1974), 
Namias (1972a), and Wyrtki (1973) are examples of the sort of 
empirical synthesis that can be achieved and should be systematically 
extended to other regions of the world and to other climatic variables. 
In these efforts, particular attention should be given to the various pos- 
sible climatic feedback processes and to the forcing functions of the 
general circulation. Here the diagnostic use of climatic models should 
prove valuable. 

Further studies should be made of the statistical characteristics of 
climatic data, both observed and simulated. Power spectrum analyses 
should be made for as many variables and locations as possible, and 
with the longest records available, as the spectrum's "redness" has an 
important bearing on questions of climatic cycles and climate prediction. 

Needed Applications of Climatic Studies 

Although closely related to the climatic data analysis and climatic 
research recommended above, the needed applications of climatic 
studies (and of climate models in particular) are so important that 
they warrant identification as a separate component of the program. 
It is in these applications that the program reaches its fruition, and 
if attention to them is delayed until our understanding is complete or 
our models perfect, they may never be undertaken. With due regard 






88 UNDERSTANDING CLIMATIC CHANGE 

for scientific caution, we believe that the time has come for a vigorous 
attack on the areas of climate model application described below. 



Simulation of the Earth's Climatic History 

The evidence presented in Appendix A (and summarized in Chapter 4) 
shows that the climatic history of the earth has been remarkably variable 
and that this history provides information that is of value in the study 
of present and possible future climates. The data assembled by paleo- 
climatologists show conclusively that the flora, fauna, and surface 
characteristics of many regions of the world have often been markedly 
different in past times than they are today. Compared with this long- 
period panorama, instrumental observations provide a frustratingly 
short record. 

It is at this juncture that the intersection of paleoclimatic and numeri- 
cal modeling studies offers the most promise: the global climatic models 
have the potential ability to simulate at least a near-equilibrium ap- 
proximation to past climates subject to the appropriate geological 
boundary conditions, while the paleoclimatic records can be used as 
verification data. Initial efforts in this direction have already begun 
(see Chapter 5), and we may expect increasing insight into the nature 
of past climates as both the models and proxy data base improve. 

In order to explore the nature of past climates systematically and 
to lay the foundation for the study of possible future climates, the fol- 
lowing studies should be made : 

The geophysical boundary conditions at a number of selected times 
in the history of the earth should be systematically assembled with a 
view toward their use in climate models. This should include global 
data on the continental land-mass positions and elevations, sea-level 
ice-sheet elevations and margins, sea-ice extent, soil type and vegeta- 
tive cover, and surface albedo. Estimates should also be made of the 
earth's rotation rate and of the solar insolation (due to orbital 
parameter changes). The selection of the time period might be based on 
criteria such as the occurrence of an ice age, the distribution of the 
continents and mountains, the opening or closing of a major oceanic 
passage, or the large-scale flooding or draining of lowlands. Periods of 
particular climatic stress such as indicated by the disappearance of 
species might also be considered. 

The various proxy records of temperature, salinity, and precipita- 
tion should also be systematically assembled for the same selected 



A NATIONAL CLIMATIC RESEARCH PROGRAM 



89 



times, to serve as verification data for the coupled climate models' simu- 
lations and as possible input or boundary conditions for uncoupled 
models. 

Dynamical global models should be used to simulate the quasi- 
equilibrium paleoclimate at selected times in the past when the boundary 
conditions external to the ocean-atmosphere system can be reasonably 
well specified. Such experiments should be focused on times when the 
global climate might be expected to be in a particularly interesting 
state (as judged from the available geological and proxy evidence) or 
when the climate might be expected to be in the process of changing 
most rapidly from one characteristic regime to another. The simulations 
should extend long enough to accumulate realistic climatic statistics 
and should use the assembled paleoclimatic data for vertification. By 
using part of the paleoclimatic evidence (namely, the sea-surface 
temperature) as a boundary condition, atmospheric gcm's may also be 
used for this purpose. 

Coupled statistical-dynamical models, or other coupled climate 
models, should be used to simulate the time-dependent climatic evolu- 
tion between the various "equilibrium" states identified above. For this 
application the dynamics of ice sheets should be incorporated into the 
coupled ocean-atmosphere models and note taken of the possible 
time dependence of the remaining boundary conditions, such as solar 
radiation and continental drift. In particular, the astronomical changes 
of seasonal radiation resulting from the variation of the earth's orbital 
parameters should be incorporated in a climate model, and the resulting 
simulated climatic changes compared with the paleoclimatic evidence. 
This recommendation parallels one made earlier in connection with the 
development of the statistical-dynamical models themselves. 

Studies should be made of possible methods to accelerate the simu- 
lation of quasi-equilibrium climatic states in the global circulation 
models, so that realistic statistics can be obtained without integration 
over long time periods. 

Exploration of Possible Future Climates 

One of the most important applications of climate models is the sys- 
tematic conduct and evaluation of climatic experiments designed to 
explore the effects of either natural or anthropogenic changes in the 
system. It is from such model-based experiments, calibrated with respect 
to observed behavior, that we must draw our conclusions as to how 
the climatic system operates and on which we should base our projec- 



90 UNDERSTANDING CLIMATIC CHANGE 

tions of likely future climates. The program in this area should include 
the determination of the global climatic effects of the following (with 
both coupled global circulation models and parameterized models): 

The changes of incoming solar radiation. These experiments should 
be performed with coupled models, in view of the dominance of the 
oceans in the planetary heat storage, and should include changes in both 
the amount and spectral distribution of solar radiation. 

The changes of land surface character and albedo, as introduced by 
deforestation, urbanization, irrigation, and changes of agricultural 
practices. 

The changes of cloudiness. These experiments should consider the 
effects of the introduction or removal of both condensation and freezing 
nuclei and the production of artificial clouds by aircraft. 

The changes of evaporation, as introduced by reservoirs, irrigation, 
and transpiration. 

The disposal of waste heat. These experiments should be made with 
coupled models and should include a broad range of rates and locations 
of heat release in both atmosphere and ocean. 

The introduction of dust and particulates into the troposphere, the 
stratosphere, or both. These experiments should consider the effects of 
scattering, absorption, fallout, and scavenging by precipitation and 
should be designed to simulate the effects of both man-made pollution 
and volcanic dust. 

The partial or complete removal of the Arctic sea ice or the Antarctic 
ice sheet. These experiments should be performed with a coupled model 
that includes the mass and heat budget of pack ice. 

The diversion of ocean currents. These experiments should be per- 
formed with coupled models. 

In climatic simulations of this kind the physical basis of each experi- 
ment should be carefully examined in order to ensure the adequacy of 
the particular model or models to be employed. The experiments sug- 
gested above are those that we believe should be performed as part of 
the climatic research program, as they involve processes or areas of 
likely maximum climatic sensitivity or changes to which the climate's 
response is relatively uncertain, and/or they represent conceivable (or 
in some cases likely) future alterations by nature or by man. 

It is important in such climatic experiments that the synoptic and 
statistical significance of the results be carefully examined. This should 
include the repetition of the experiment under slightly different (but 
admissible) conditions to determine its stability and noise level and the 



A NATIONAL CLIMATIC RESEARCH PROGRAM 91 

analysis of independent simulations with other models. Only in this way 
can we hope to accumulate the necessary experimental knowledge on 
which to base our expectations of future climatic states. This, together 
with the knowledge gained from the observational and research por- 
tions of the program outlined above, will lay the scientific foundation 
for what might be called climatic engineering. 



Development of Long-Range or Climatic Forecasting 

A third important area of application of climatic studies is the problem 
of long-range or climatic forecasting on time scales of months, seasons, 
and years. There have been numerous studies of this question almost 
since the beginning of recorded observations. This research has not 
solved the problem but has at least identified some of its ingredients. 
We believe that further efforts should be made to systematically acquire 
the data and perform the research necessary to attack this problem 
anew, especially with the aid of climatic models. 

Clearly the demand for climatic or long-range forecasts greatly 
exceeds present capability. An accurate prediction of the temperature 
or rainfall anomaly over, say, the central plains of North America or 
over the Ukraine a decade, a year, or even a season in advance would be 
of great value. And even a somewhat less accurate (but reliable) 
prediction of the likelihood of such anomalies would be of great use to 
those involved in agriculture, energy supply allocation, and commerce. 
At present, the skill of the experimental long-range outlooks prepared 
by the National Weather Service for the 30-day temperature anomaly 
at some 100 U.S. cities is only 11 percent greater than chance and only 
2 percent greater than chance for the 30-day precipitation anomaly. 
These forecasts are principally prepared by a mixture of empirical and 
statistical methods and have also been applied to the seasonal predic- 
tion of temperature (Namias, 1968). 

The ability of numerical models to perform useful long-range or 
climatic forecasting (i.e., forecasts over monthly, seasonal, or annual 
periods) has not been systematically examined because of the large 
amounts of computation involved and the unavailability of suitable 
models. Such efforts must also contend with the crucial questions of 
climatic predictability, noted in Chapter 3, and the long-range stability 
of the models themselves. We believe that further attention should be 
given to these problems, using the expanded data base, the coupled 
dynamical models, and the new computer resources called for in the 
climatic program. We therefore recommend that 



92 UNDERSTANDING CLIMATIC CHANGE 

The coupled global circulation models should be systematically ap- 
plied to the preparation of a series of long-range forecasts using ob- 
served initial conditions wherever possible. These integrations should 
extend over at least several seasons, well beyond the limit of local 
predictability. Appropriate climatic statistics should be drawn from these 
integrations and systematically compared with the observed variations 
of all the climatic elements available and statistically analyzed for pos- 
sibly significant trends of regional climatic anomalies. 

The statistical-dynamical models and other appropriate members 
of the parameterized climate model hierarchy should be used in the 
preparation of similar long-range forecasts. 

Systematic empirical and diagnostic studies of longer-period varia- 
tions in the climatic system should be undertaken with the aid of models 
and the expanding data base of monitored variables. 

Assessment of Climate's Impact on Man 

While the above efforts are concerned with the physical aspects of the 
problem of climatic variation, a climatic research program should also 
include studies of the impact of climate and climatic change on man 
himself; this is best done with the guidance and insight provided by 
climate models. While many studies have been made in this important 
area, such as those of the Department of Transportation's Climatic 
Impact Assessment Program (ciap), more comprehensive research 
should be undertaken on a long-term basis. These studies may be 
characterized as seeking answers to such questions as "What is a 
1 -degree change of mean winter temperature worth, after all?" or even 
"Climatic variation: so what?" The study of the impacts of climatic 
variations on man is also a way of establishing priorities for research. 

Climate and Food, Water, and Energy 

That climate has a dominant influence on agricultural food production, 
water supply, and the generation and use of energy is generally recog- 
nized. The kinds and amounts of crops that may be grown in various 
regions, the water available for domestic, agricultural, and industrial 
use, and the consumption of electrical energy and fossil fuels all depend 
in large measure on the distribution of temperature, rainfall, and sun- 
shine. During the global warming of the first part of this century, for 
example, the average length of the growing season in England (as 
measured by the duration of temperatures above 42 °F) increased by 



A NATIONAL CLIMATIC RESEARCH PROGRAM 93 

two to three weeks and during the more recent cooling trend since the 
1940's has undergone a comparable shortening (Davis, 1972). Al- 
though Maunder (1970), Johnson and Smith (1965), and others have 
surveyed the vast literature on the effects of climatic change on man, 
further quantification of these effects is needed, particularly as a func- 
tion of the time and space scales of atmospheric variability. Accordingly, 
we recommend that research be devoted to the following: 

The systematic assembly from both national and international sources 
of data on worldwide food production and the analysis of their re- 
sponse and sensitivity to variations of climate on monthly and seasonal 
time scales. Such analyses should then be used to model or simulate 
the total agricultural response to hypothetical climatic variations. We 
note that in some cases it may be the variance or extremes of climate, 
rather than the averages themselves, that will prove to be the more 
important factor. An applied systems study of this problem has been 
recently initiated by R. A. Bryson and colleagues at the University of 
Wisconsin, with the aim of developing predictive relationships between 
climate and food supply, which will be useful for policy decisions. 

The systematic assembly of worldwide data on available water supply, 
both from rainfall and snowpack, and its patterns of use and loss. 
Analysis should then be undertaken of the water supply system's re- 
sponse and sensitivity to variations of climate and simulation models 
constructed. 

The systematic assembly of worldwide data on the production and 
use of energy and the determination of its response and sensitivity to 
climatic variations. As in the cases of food and water, simulation models 
should be constructed, so that the consequences of various patterns 
of hypothetical climatic change can be estimated. 

Social and Economic Impacts 

Although it is difficult to obtain useful measures of the social and eco- 
nomic impacts of climatic change, increased attention should be given 
to this aspect of the problem. This is a problem in which the "noise 
level" of nonclimatic factors is very high and for which the physical 
scientist's knowledge must be supplemented by the skills and methods 
of social and political scientists. The goal of this research should be the 
development of an overall model of societal response to climatic change. 
This is an area in which international cooperation should be sought, 
and efforts such as those now being proposed by the International 



94 UNDERSTANDING CLIMATIC CHANGE 

Federation of Institutes of Advanced Study should be supported and 
expanded. 



THE PUN 

Our recommendations for the planning and execution of the climatic 
research program outlined above are given here in terms of what we 
believe to be the appropriate subprograms, the necessary facilities and 
support, and the desirable timetable for both the short-range and long- 
range phases. We also offer some observations on the program's ad- 
ministration and coordination, although we recognize that a program 
of this scope will require much further planning and that the support 
and cooperation of many persons and agencies will be necessary for its 
successful execution. 



Subprogram Identification 

In a program as broad as that envisaged here, it is convenient to think 
in terms of a number of components or subprograms, each concerned 
with a specific portion of the overall effort. Such subprograms also 
represent the necessary division of effort for the practical execution of 
the program. The ncrp itself should ensure the coordination of the 
various subprograms and maintain an appropriate balance of effort 
among them. 

Climatic Data- Analysis Program (CDAP) 

In order to promote the extensive assembly and analysis of climatic 
data outlined above, we recommend that a Climatic Data-Analysis Pro- 
gram (cdap) be established as a subprogram of the ncrp. The purposes 
of this subprogram are to facilitate the exchange of data and informa- 
tion among the various climatic data depositories and research projects 
and to support the coordinated preparation, analysis, and dissemination 
of appropriate climatic statistics. 

Climatic Index Monitoring Program (CI MP) 

In order to promote the monitoring of the various climatic indices out- 
lined above, we recommend that a Climatic Index Monitoring Program 
(cimp) be established as a second subprogram of the ncrp. The pur- 
poses of this subprogram are to support and coordinate the collection of 



A NATIONAL CLIMATIC RESEARCH PROGRAM 95 

data on selected climatic indices and to ensure their systematic dis- 
semination on a timely and sustained basis. 



Climatic Modeling and Applications Program {CM A?) 

In order to promote the construction and application of the climatic 
models outlined above, we recommend that a Climatic Modeling and 
Applications Program (cmap) be established as a third subprogram of 
the ncrp. The purposes of this subprogram are to support and co- 
ordinate the development of a broad range of climatic models, to sup- 
port necessary background scientific research, and to ensure the sys- 
tematic application of appropriate models to the problems of climatic 
reconstruction, climatic prediction, and climatic impacts. 



Facilities and Support 

The availability of adequate facilities and support and the design of 
coordinating mechanisms are necessary to carry out the various sub- 
programs recommended for the ncrp and should be given careful con- 
sideration. Of primary importance are the roles of climatic data-analysis 
facilities and research consortia, the needed high-speed computers, and 
the required levels of funding. 

Climatic Data- Analysis Facilities 

To assist in the implementation of both the Climatic Data Analysis Pro- 
gram (cdap) and Climatic Index Monitoring Program (cimp), we 
recommend the development of new climatic data-analysis facilities at 
appropriate locations, including linkage to the various specialized data 
centers and climatic monitoring agencies by a high-speed data-trans- 
mission network. Such facilities should have access to machines of the 
highest speed and capacity available and be staffed by specialists in 
data analysis, transmission, and display. Collection of certain climatic 
data by a group of specialized facilities appears more desirable than 
does collection of all data by a single centralized facility. 

We envisage these facilities as performing the bulk of the recom- 
mended cdap. This would include the inventory, compilation, processing, 
analysis, and documentation of both conventional and proxy climatic 
data. Close working cooperation is envisaged with specialized data 
depositories; for conventional atmospheric and oceanic data these 
include noaa's National Climatic Center and National Oceanographic 



96 UNDERSTANDING CLIMATIC CHANGE 

Data Center, for satellite data the National Environmental Satellite 
Service, for glaciological data the Geological Survey's Data Center A in 
Tacoma, for ice-core data the Army's Cold Regions Research and Engi- 
neering Laboratory, for marine cores Columbia University's Lamont- 
Doherty Geological Observatory, and for pollen and tree-ring data the 
universities of Wisconsin and Arizona. 

We also envisage the data-analysis facilities as playing a prominent 
role in the cimp and in the processing, analysis, and dissemination of the 
results on as nearly a real-time basis as possible. Certain of the facilities 
could serve as global climatic "watchdogs" and might have a resident 
scientific staif to perform diagnostic research as appropriate. 

Climatic Research Consortia and Manpower Needs 

We envisage the broad range of research and analysis recommended here 
as being best performed by a number of institutions and groups. This 
is desirable in order to ensure the breadth of viewpoint and diversity 
of approach necessary in a problem as close to the unknown as is cli- 
matic variation. An attempt to carry out all the recommended activities 
and research by a single institution would in any case be a practical 
impossibility. 

Research on climate and climatic variation at the present time is 
principally performed in governmental laboratories and in a variety of 
research projects in universities and other institutions, usually with the 
support of the federal government. Chief among the laboratories con- 
cerned with elements of the climatic problem are noaa's Geophysical 
Fluid Dynamics Laboratory, noaa's National Environmental Satellite 
Service and Environmental Data Service, nsf's National Center for 
Atmospheric Research, and nasa's Goddard Institute for Space Studies. 
More specialized research on problems related to climate is also per- 
formed by the U.S. Geological Survey and by the operational services 
and laboratories of the U.S. Army, Navy, and Air Force. Many of 
the climate-related research projects in universities and other institu- 
tions are supported by the National Science Foundation through its 
programs for atmospheric, oceanic, and polar research; by dot's Cli- 
matic Impact Assessment Program; and by arpa's Climate Dynamics 
Program. These include the various Quaternary research groups, geo- 
logical and oceanographic laboratories, numerical modeling groups, and 
polar studies and environmental institutes. 

Each of these efforts makes a contribution to the national climatic 
research picture, and they represent a valuable reservoir of experience 



A NATIONAL CLIMATIC RESEARCH PROGRAM 97 

and talent. In order to promote greater cooperation and exchange, to 
ensure an appropriate balance of effort, and to give such research the 
needed stability and coherence, we recommend that efforts be made to 
coordinate present research more effectively as parts of a national 
climatic research program. We believe that this can be achieved best by 
the formation of cooperative associations of existing climatic research 
groups and the initiation of whatever new research efforts may be re- 
quired as parts of such associations. We accordingly recommend the 
formation of a number of climatic research consortia among various 
research groups as appropriate to their interests, with each such con- 
sortium having links to computing facilities of the highest speed and 
capacity available. Such research consortia would serve as valuable 
coordinating mechanisms for the broad range of climatic research en- 
visaged in the Climatic Modeling and Applications Program (cmap), 
as well as giving both coherence and flexibility to the ncrp as a whole. 
The present mode, norpax, and climap programs may serve as useful 
examples for such consortia. As the national program develops, the 
possible need for new institutional structures or facilities should be- 
come clear. Our recommendations reflect the consensus that maximum 
use should be made of existing institutions while further consideration 
is given to the possible need for their expansion. 

Aside from institutional arrangements, however, we believe that the 
proposed research program unquestionably calls for the initiation and 
support of new mechanisms to provide an expanded base of appro- 
priately trained scientific and technical manpower. We accordingly 
recommend that programs for technical training be developed and that 
both predoctoral and postdoctoral fellowships in the broad area of 
climatic research be established as soon as possible. 



Computer Requirements 

The required access to high-speed computers has been alluded to several 
times in the discussion of the recommended program. Although it is 
difficult to make precise projections, the volume of data processing 
involved in the analysis and monitoring portions of the program alone 
indicate that a dedicated machine of at least the cdc 7600 class is 
required for the implementation of the cdap and cimp. The computer 
needs of the research consortia and of the other research groups involved 
in the modeling portions of the program are even more demanding, in 
view of the variety. of the needed climatic models and tests and the 
number and the length of the necessary climatic simulation experiments 



98 UNDERSTANDING CLIMATIC CHANGE 

and applications. Our estimates of the ncrp's overall computer require- 
ments are given in Table 6.2 and call for a very significant increase over 
present levels of computer usage. 

If anything, these estimates may be too low. In its computer planning, 
ncar has estimated a climate-related usage of several cdc 7600 units 
by 1980 for the needs of ncar and the university community it serves 
(W. M. Washington, personal communication), while the installation of 
the ti-asc system at gfdl in 1974 will likely significantly raise their 
machine usage for climatic studies. As shown in Table 6.2, it is estimated 
that climatic data analysis and monitoring will require the full-time use 
of at least one fourth-generation machine, and that climatic modeling 
and applications will require the full-time use of at least one fifth-genera- 
tion machine. We therefore recommend that machines of the cdc-7600 
class be secured as soon as possible for the use of the data-analysis 
facilities and the associated elements of the cdap and cimp and that 
planning begin for the acquisition of computers of the ti-asc or illiac-4 
class for the use of the climatic research consortia and the associated 
elements of the cmap. It will also be necessary to provide broadband 
communication links among the various facilities and cooperating groups 
and with the climatic research community as a whole. 



TABLE 6.2 Estimated Computing Needs for the National Climatic 
Research Program ° 



Present Use * Projected Use 

Climatic data analysis and monitoring 
Atmospheric gcm development 
Oceanic gcm development 
Coupled gcm's (climate models) 

Development and tests 

Climatic reconstructions 

Climatic experiments and projections 
Other models and studies 

total 1.5 16.5 



" In units of cdc 7600 years. 

6 Estimated 1973 national total, exclusive of operational agencies. 

c For the program year circa 1980. 

d Envisaged as use by climatic data-analysis facilities. 

e Estimating 0.2 usage at ncar, 0.5 usage at gfdl, and 0.1 total usage elsewhere. 

f Envisaged as use by cooperative climatic research consortia. 



0.2 


1.5* 


0.8 c 


3.0 


0.2 


2.0 


0.1 


3.0' 


-0 


2.0' 


0.1 


3.0 f 


0.1 


2.0 



A NATIONAL CLIMATIC RESEARCH PROGRAM 99 

Estimated Costs 

The cost of the recommended national climatic research program is 
difficult to determine accurately without a great deal of information on 
observational, computing, and support costs from the various agencies 
and institutions presently engaged in the many aspects of climatic re- 
search. Rather than seeking such detailed data, we have restricted our- 
selves to gross projections on the basis of estimates of the costs of 
present efforts. Our estimates of the expenditures for climatic research 
{not including the costs of instruments, observing platforms, or opera- 
tional and service-related activities) are given in Table 6.3. Our pro- 
jections of the growth of these (direct) costs during the early phases 
of the program (i.e., to the year 1980) are shown in Figure 6.1, along 
with the percentage increases over the preceding year; these estimates, 
of course, depend directly on the base figures that are used and are sub- 
ject to further refinement. These figures are intended for order-of- 
magnitude guidance only and will require revision as the program 
develops. 

We recognize that the ultimate distribution of resources among the 
various subprograms of the ncrp will be determined by the sense 
of priorities of the government and by the capabilities of the research 
community. The estimates shown in Figure 6.1 for the year 1980 are 
based on our preception of the needed increases over present efforts in 
the areas of data analysis and monitoring (cdap and cimp), especially 
those concerning satellite data and the monitoring of oceanic climatic 
indices. In the area of climatic modeling and applications (cmap), 
the largest increases over present efforts are envisaged for the develop- 



TABLE 6.3 Estimated Expenditures for Climatic Research" (in $10 6 /yr) 

Present Projected 

(1974) (c.1980) 

Climatic data assembly and analysis 
Climatic index monitoring h 
Climatic modeling and applications 

~18 67 

° Based on estimates of the climate-related research sponsored by the nsf, dot, and dod and that 
conducted by gfdl, ncar, nasa, and noaa but not including essentially operational or service- 
related activities. 
b Not including costs of instruments or observing platforms. { 



5 


18 


4 


12 


9 


37 



100 

70 



UNDERSTANDING CLIMATIC CHANGE 



60 - 



50 - 



o 40 



o on - 



30 



20 - 



10 



- 














67 


61 










51 




CMAP 


- 














39 


- 















28 


_ 












CIMP 
CDAP 


22 


18 




(20%) 


(30%) 


| (40%) 


(30%) 


(20%) 


(10%) 



1975 



1976 



1977 



1978 



1979 



1980 



1974 
(present) 
FIGURE 6.1 Projected costs of the National Climatic Research Program (NCRP). The 
numbers in parentheses are the percent increase over the preceding year's expenditures. 



ment and application of coupled global climate models and climatic 
impact studies. The relatively rapid growth rate during the program's 
third and fourth years are projected to include the acquisition of the 
necessary computers and networks. Overall, the recommended pro- 
gram calls for an approximate fourfold expansion of the support of 
research on climatic variation by the year 1980; the program's costs 
beyond this time are more difficult to estimate and will depend on the 
progress and opportunities developed prior to that time. 

It is useful to compare these cost projections with the direct and 
indirect costs of present garp efforts and those of closely related pro- 
grams. In fiscal year 1973 the direct garp expenditures totaled $13.2 
million, about 54 percent of which represented expenditures by the De- 



A NATIONAL CLIMATIC RESEARCH PROGRAM 



101 



partment of Commerce and nasa directed toward the improvement of 
weather forecasting, with the remainder expended by nsf for research 
on both forecasting and general circulation studies. Some of these costs 
are included in the estimates in Table 6.3, insofar as they can be identi- 
fied as directed toward climatic research. The indirect costs associated 
with garp amounted to $29.0 million in fiscal year 1973 and are not 
reflected in the present climatic research estimates. 

In addition to these efforts, there are other current programs that 
contribute to garp and whose costs should not be overlooked. The 
implementation of the World Weather Watch (www) and its satellite 
system represented $1.5 million direct costs and $54.5 million indirect 
costs in fiscal year 1973, while systems design and technological de- 
velopment represented $2.4 million direct costs and $50.1 million in- 
direct costs in the same period. The extent to which elements of the 
recommended cdap and cimp subprograms of the ncrp may be con- 
sidered as add-ons to such existing programs needs further considera- 
tion, as does the extent to which the future costs of garp itself may be 
merged with those envisaged for the ncrp. 

Also in need of further study are the United States' contributions 
to the costs of the various subprograms recommended as part of the In- 
ternational Climatic Research Program (icrp) described below, as well 
as the impacts of inflation. We also note that funds will be required for 
the training of additional scientific manpower in all aspects of the 
research program. 

Timetable and Priorities within the Program 

We recognize the need for flexibility in a research program of this 
kind, and that future technological and research discoveries may have 
important impacts on the direction of climatic research. In spite of 
these unknown factors, however, some consideration of goals and 
priorities is useful. Here we present our recommendations for the ob- 
jectives of the initial phase of the program (1974-1980) and the 
necessary sequence of planning activities for both these goals and those 
of the long-term phase (1980-2000). Our recommendations for a 
coordinated international program are considered subsequently. 



The Initial Phase (1974-1980) 

Once the decision is made to develop a national climatic research pro- 
gram, we recommend that planning begin immediately for the implemen- 
tation of its component activities and subprograms. Our specific recom- 



102 



UNDERSTANDING CLIMATIC CHANGE 



mendations for both the immediate and subsequent objectives during 
this phase of the program are shown in Table 6.4 in terms of the data- 
analysis, index-monitoring, and modeling subprograms identified earlier. 
Here our sense of relative priorities is given implicitly by the ranking 
into immediate and subsequent objectives; these time scales refer to 
the expected times of the achievement of first useful results, with the 
recognition that initial development must in some cases begin earlier 



TABLE 6.4 Goals for the Initial Phase of the NCRP (1974-1980) 





Immediate Objectives 


Subsequent Objectives 


Subprogram 


(1974-1976) 


(1 


976-1980) 


Climatic data 


1. 


Development of climatic data- 


1. 


Development of global 


analysis 




analysis facilities 




climatic data-analysis 


(cdap) 


2. 


Statistical analysis of climatic 




system (fgge) 






variability, predictability, 


2. 


Assembly and process- 






feedback processes 




ing of global climatic 




3. 


Statistical climatic-impact 
studies (crops, human affairs) 


3. 


data (conventional, 
satellite, historical, 
proxy data) 
Development of cli- 
matic impact models 


Climatic index 


1. 


Monitoring of oceanic mixed- 


1. 


Satellite monitoring of 


monitoring 




layer 




global heat-balance 


(cimp) 


2. 


Monitoring of ice, snow, and 




components 






cloud cover 


2. 


Monitoring selected 




3. 


Expansion of proxy data 
sources 




physical processes 
(fgge) 




4. 


Monitoring system simulation 
studies 


3. 


Development of global 
climatic index monitor- 
ing system 


Climatic model- 


1. 


Development of oceanic 


1. 


Development of fully 


ing and appli- 




mixed-layer models 




coupled atmosphere- 


cations 


2. 


Development and analysis of 




ocean-ice gcm's 


(cmap) 




provisionally coupled gcm's 
(sensitivity, predictability 
studies) 


2. 


Development of statis- 
tical-dynamical cli- 
mate models 




3. 


Development of simplified cli- 
matic models and related 
theoretical studies 


3. 


Parameterization of 
mesoscale processes, 
simulation of climatic 




4. 


Selected paleoclimatic recon- 
structions 


4. 


feedback mechanisms 
(fgge) 

Experimental seasonal 
climatic forecasts by 
dynamical models 



A NATIONAL CLIMATIC RESEARCH PROGRAM 103 

and that further development and application will continue later. This 
ranking also reflects a balance between the relative ease of accomplish- 
ment and the relative potential for initial practical usefulness. We 
believe that progress toward the subsequent objectives will require 
the support of all immediate objectives of the program, with new 
priorities evolving as a function of achievement and opportunity. 



Relationship to the FGGE (1978-1979) 

The First garp Global Experiment (fgge), now planned for 1978- 
1979, is primarily an attempt to collect a definitive global data set for 
use in the improvement of weather prediction by numerical atmospheric 
models. The potential value' of these data for climatic research lies not 
so much in their display of seasonal and interhemispheric variations, 
valuable as that will be, but in the fact that many of the short-period 
physical processes to be intensely measured or parameterized in fgge 
are also important for the understanding of climate. Among these are 
the processes of convection, boundary-layer dynamics, and the at- 
mosphere's interaction with the surface of the ocean. 

The observational requirements during the fgge call for measurement 
of the atmospheric temperature, water vapor, cloud cover and eleva- 
tion, wind, and surface pressure, together with the surface boundary 
variables of sea-surface temperature, soil moisture, precipitation, snow 
depth, and sea-ice distribution. To enhance their value for climatic 
studies, we recommend that these data be supplemented during fgge 
insofar as possible by observations of the global distributions of ozone, 
particulates, surface and planetary albedo, incoming solar and outgoing 
terrestrial radiation, vegetal cover, and the continental freshwater 
runoff. We recommend that special observations also be made in con- 
junction with regional programs, such as norpax and polex, which 
are expected to be in operation during the fgge. 

The Long-Term Phase (1980-2000) 

The long-range goals and full-scale operation of the ncrp in the period 
beyond 1980 are portrayed in the upper part of Figure 6.2. During this 
period, the full interaction among the observational, analysis, modeling, 
and theoretical components of the program will occur, leading to the 
development of an operational global climatic data system and, it is 
hoped, to the acquisition of an increasingly accurate theory of climatic 
variation. Although priorities cannot be set at such long range, the 
eventual practical payoffs of this program will be the determination of 























i 












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c — 










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o o 










c .r c 




«. "5 ** 










2 « tt 
*-• c !5 

«,E 3 

8 o £ 

5 ^ 




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104 



A NATIONAL CLIMATIC RESEARCH PROGRAM 105 

the degree to which climatic variations on seasonal, annual, decadal, 
and longer times scales may be predicted and the degree to which they 
may be controlled by man. 



Administration and Coordination 

The administrative structure and coordination of the recommended 
program are the responsibility of the federal government and were not 
given extensive consideration. However, noting the concern with the 
problem of climatic variation in many parts of the government, and the 
widespread participation of many governmental and nongovernmental 
groups in climatic research, we believe that the program should be ad- 
ministered in such a way that the interests of all are effectively repre- 
sented and coordinated. It is particularly important that the advice of 
the scientific community be used in the design and development of 
the major elements of the research program. 

Both the short-term and long-term goals of the ncrp are also shared 
by the International Climatic Research Program (icrp) recommended 
below. The development of this international program should proceed 
in parallel with the ncrp and should be closely coordinated with garp. 
The principal activities within garp up to the present time have focused 
on the problem of improving the accuracy and extending the range of 
weather forecasts, and the United States' contributions to garp in par- 
ticular have emphasized the development and use of numerical models 
for this purpose. These efforts are necessary steps in the development 
of an adequate modeling capability for both weather prediction and 
climate, and were they not already under way as part of garp they 
would have had to be undertaken through some other means as a 
prelude to the climatic research program. 

A COORDINATED INTERNATIONAL CLIMATIC RESEARCH 
PROGRAM (ICRP) 

Many of the efforts envisaged within the ncrp are of an obvious inter- 
national character, and the degree to which these should be regarded 
as national as opposed to international activities is not of critical im- 
portance for our purposes. The important point is that there are inter- 
national efforts now under way within garp of direct relevance to the 
climatic problem, of which we note especially the International Study 
Conference on the Physical Basis of Climate and Climate Modeling 
held in Sweden in July and August 1974 under the auspices of the 
icsu/wmo garp Joint Organizing Committee. The recommendations 



106 UNDERSTANDING CLIMATIC CHANGE 

and programs resulting from this and subsequent planning conferences 
should be closely coordinated with the U.S. national program. We offer 
here our recommendations for an appropriate international climatic re- 
search program and some observations on how such a program might 
best be coordinated with garp itself. 



Program Motivation and Structure 

The observational programs planned in support of garp offer an un- 
paralleled opportunity to observe the global atmosphere, and every 
effort should be made to use these data for climatic purposes as well 
as for the purposes of weather prediction. The climatic system, how- 
ever, consists of important nonatmospheric components, including the 
world's oceans, ice masses, and land surfaces, together with elements 
of the biosphere. While it is not necessary to measure all of these com- 
ponents in the same detail with which we observe the atmosphere, their 
roles in climatic variation must not be overlooked. 

In addition to the fundamental physical differences discussed in 
Chapter 3, the problem of climatic variation also differs from that 
of weather forecasting by the nature of the data sets required. The 
primary data needs of weather prediction are accurate and dense 
synoptic observations of the atmosphere's present (and future) states, 
while the data needed for studies of climatic variation are longer-term 
statistics of a much wider variety of variables. When climatic variations 
over long time scales are considered, these variables must be supplied 
from fields outside of observational meteorology. Thus, an essential 
characteristic of climate studies is its involvement of a wide range of 
nonatmospheric scientific disciplines. 

The types of numerical models needed for climatic research also 
differ from those of weather prediction. The atmospheric gcm's (which 
represent the ultimate in weather models ) do not need a time-dependent 
ocean for weather-forecasting purposes over periods of a week or two. 
For climatic change purposes, on the other hand, such numerical models 
must include the changes of the oceanic heat storage. Such a slowly 
varying feature may be regarded as a boundary or external condition 
for weather prediction but becomes an internal part of the system 
for climatic variation. 



International Climatic Research and GARP 

In view of these characteristics, we suggest that while the garp concern 
with climate is a natural one, as indicated above the problem of climate 



A NATIONAL CLIMATIC RESEARCH PROGRAM 107 

goes much beyond the present basis and emphasis of garp. Accordingly, 
we recommend that the global climate studies that are under way within 
garp be viewed as leading to the organization of a new and long-term 
international program devoted specifically to the study of climate and 
climatic variation, which we suggest be called the International Climatic 
Research Program (icrp). 



International Climatic Decades (1980-2000) 

We suggest that the observational programs of garp, and especially 
those of the fgge, be viewed as preliminary efforts, later to be expanded 
and maintained on a long-term basis. In particular, we recommend 
that the special data needs of climatic studies be supported on an inter- 
national scale through the designation of the period 1980-2000 as the 
International Climatic Decades (icd), during which intensive efforts 
would be made to secure as complete a global climatic data base as 
possible. 

The general outline of the envisaged international program (icrp) 
is sketched in the lower part of Figure 6.2, and the program's scientific 
elements are discussed in more detail below. 



Program Elements 

Climatic Data Analysis 

The main thrust of the international climatic program should be the 
collection and analysis of climatic data during the icd's, 1980-2000. 
During this period, the participation of all nations should be sought in 
order to develop global climatic statistics for a broad set of climatic 
variables. We urge that these efforts include international cooperation 
in the systematic summary of all available meteorological observations 
of climatic value, including oceanographic observations in the waters 
of coastal nations. 



International Paleoclimatic Data Network (IPDN) 

We urge the development of an international cooperative program for 
the monitoring of selected climatic indices and the extraction of histori- 
cal and proxy climatic data unique to each nation, such as indices of 
glaciers, rain forest precipitation, lake levels, local desert history, tree 
rings, and soil records. Specifically, we recommend that this take the 
form of an International Paleoclimatic Data Network (ipdn), as a 



108 UNDERSTANDING CLIMATIC CHANGE 

subprogram of the icrp. The cooperation of such organizations as 
scar, scor, and the International Union for Quaternary Research 
(inqua) should be sought in this program. 

The contents of these international observational efforts might 
possibly broadly follow those recommended for the U.S. national effort, 
with modifications as appropriate to each nation's needs and capabilities. 
In addition, we recommend that the icrp undertake the following: 

The international collection of special climatic data sets on such 
events as widespread drought and floods and following major environ- 
mental disturbances such as volcanic eruptions; 

Programs to encourage international exchange of climatic data and 
analyses. 

Climatic Research 

Although cooperative research studies are desirable, we recognize that 
the large-scale numerical simulation of climate with cgcm's can now 
be carried out in only a relatively few countries. To promote wider 
international participation in climatic research, we therefore recommend 
that the icrp include the following: 

Programs and activities to encourage international cooperation in 
climatic research and to facilitate the participation of developing na- 
tions that do not yet have adequate training or research facilities. 

Internationally supported regional climatic studies in order to describe 
and model local climatic anomalies of special interest. 

The contents of these and other research activities of the icrp might 
also broadly follow those recommended for the U.S. national effort, 
with appropriate modifications for each nation's interests and capabilities. 



Global Climatic Impacts 

While all nations are tied in some fashion to the world pattern of 
climate, some are more vulnerable to climatic variations than others by 
virtue of their locations and the delicacy of their climatic balance. We 
therefore recommend that the icrp include the following: 

International cooperative programs to assess the impacts of observed 
climatic changes on the economies of the world's nations, including 



A NATIONAL CLIMATIC RESEARCH PROGRAM 109 

the effects on the water supply, food production, and energy utilization. 
This should include the impacts of variations of oceanic climate for 
those nations whose economies are dependent on the sea. The coopera- 
tion of appropriate international agencies of the United Nations and of 
other groups such as the International Federation of Institutes of Ad- 
vanced Study should be sought. 

Cooperative analyses of the regional impacts of possible future cli- 
mates. Such studies could be of great importance to many countries, 
particularly emerging nations making long-range policy decisions con- 
cerning the development of their resources. 

Program Support 

The question of the details of support of the icrp was not dealt with. 
It seems clear, however, that an appropriate balance of effort should be 
maintained among icrp, the various national climatic research pro- 
grams, and other international programs such as the World Weather 
Watch (www) and the United Nations Environment Program (unep). 
The services of groups performing the function of the present garp 
Joint Organizing Committee and its Joint Planning Staff will also be 
necessary for the success of the international program. 

In order to assist in the coordination of the icrp, we urge that sup- 
port be made available by the appropriate agencies of the United Na- 
tions on a scale commensurate with the breadth and importance of the 
problem. This should include a budget adequate for the effective inter- 
national coordination of the icrp on a scale significantly greater than 
that of garp and on a continuing long-term basis. We also urge that 
scientific assistance be sought from the International Council of Scientific 
Unions in support of selected icrp subprograms. 



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APPENDIX A 
SURVEY OF PAST CLIMATES 



INTRODUCTION 

The earth's climates have always been changing, and the magnitude 
of these changes has varied from place to place and from time to time. 
In some places the yearly changes are so small as to be of minor inter- 
est, while in others the changes can be catastrophic, as when the 
monsoon fails or unseasonable rain delays the planting and harvesting of 
basic crops. On a longer time scale, certain decades have striking 
and anomalous characteristics, such as the severe droughts that affected 
the American Midwest during the 1870's, 1890's, and 1930's and the 
high temperatures recorded globally during the 1940's. And on still 
longer time scales, the climatic regimes that dominated certain centuries 
brought significant changes in the global patterns of temperature, rain- 
fall, and snow accumulation. For example, northern hemisphere winter 
temperatures from the midfifteenth to the midnineteenth centuries 
were significantly lower than they are today. The late nineteenth century 
represented a period of transition between this cold interval — sometimes 
known as the Little Ice Age — and the thermal maximum of the 1940's. 
Some idea of the magnitude of the climatic changes that characterized 
the Little Ice Age can be gained from a study of proxy or natural 
records of climate, such as those of alpine glaciers. As shown in Figure 
A.l, as late as the midnineteenth century the termini of these glaciers 
were still advanced well beyond their present limits. 

The practical as well as the purely scientific value of understanding 

127 



128 



UNDERSTANDING CLIMATIC CHANGE 





FIGURE A.l The Argentiere glacier in the French Alps, (a) An etching made about 1850, 
showing the extent of the glacier during the waning phase of the Little Ice Age. (b) Photo- 
graph of the same view taken in 1966. [From LeRoy Ladurie (1971).] 



APPENDIX A 129 

the processes that bring about climatic change is self-evident. Only by 
understanding the system can we hope to comprehend its past and to 
predict its future course. This objective can be achieved only by study- 
ing the workings of the global climate machine over a time span ade- 
quate to record a representative range of conditions in nature's own 
laboratory, and for this the record of past climates is indispensable. 

From the evidence discussed below and summarized in Figure A. 2 
we conclude that a satisfactory perspective of the history of climate 
can be achieved only by the analysis of observations spanning the entire 
time range of climatic variation, say, from 10" 1 to 10° years. Near the 
short end of this range there is a rich instrumental record to collate and 
analyze, although as discussed elsewhere in this report, awkward gaps 
exist in our knowledge of many parts of the air-sea-ice system during 
even the past hundred years. As the time scale of observations is 
lengthened to include earlier centuries, the direct instrumental record 
becomes less and less adequate. A continuous time series of observa- 
tions as far back as the seventeenth century is available for only one 
area. For earlier times the instrumental record is blank, and indirect 
means must be found to reconstruct the history of climate. 

The science of paleoclimatology is concerned with the earth's past 
climates, and that branch which seeks to map the reconstructed climates 
may be referred to as paleoclimatography. So defined, the science of 
paleoclimatology does far more than satisfy man's natural curiosity 
about the past; it provides the only source of direct evidence on pro- 
cesses that change global climate on time scales longer than a century. 
When calibrated and assembled into global arrays, these data will be 
essential in the reconstruction of paleoclimates with numerical models. 

Nature of Paleoclimatic Evidence 

The subject of ancient climates may conveniently be approached in 
terms of the nature of the climatic record, whether from human 
(historical) recordings or from proxy or natural climatic indicators. It 
is therefore convenient to identify historical climatic data and proxy 
climatic data as sources of paleoclimatic evidence. 

Prior to the period of instrumental record, historical climatic data 
are found in books, manuscripts, logs, and other documentary sources 
and provide valuable (although fragmentary) climatic evidence before 
the advent of routine meteorological observations. Lamb (1969) has 
pioneered the collection of such data and has charted the main course 
of climate over Western Europe during the past 1000 years [Figure 
A.2(b)]. Where the historical or manuscript record overlaps the instru- 



130 



UNDERSTANDING CLIMATIC CHANGE 



I960 



AIR TEMPERATURE 
COLD WARM COLO 

1900 




< 

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or 

< 

LU 

VI880 

.2 .4 .6 
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the five-year average surface temperatures over the region 0-80 °N during the last 100 years 
(Mitchell, 1963). (b) Winter severity index for eastern Europe during the last 1000 years 
(Lamb, 1969). (c) Generalized midlatitude northern hemisphere air-temperature trends 
during the last 15,000 years, based on changes in tree lines (LaMarche, 1974), marginal 
fluctuations in alpine and continental glaciers (Denton and Karlen, 1973), and shifts in 
vegetation patterns recorded in pollen spectra (van der Hammen et al., 1971). (d) Gen- 
eralized northern hemisphere air-temperature trends during the last 100,000 years, based 
on midlatitude sea-surface temperature and pollen records and on worldwide sea-level 
records (see Figure A.13). (e) Fluctuations in global ice-volume during the last 1,000,000 
years as recorded by changes in isotopic composition of fossil plankton in deep-sea core 
V28-238 (Shackleton and Opdyke, 1973). See legend for identification of symbols (1) 
through (6). 



APPENDIX A 131 

mental record, the climatic reconstructions may be confirmed and 
calibrated by the latter. 

In contrast, the proxy record of climate makes use of various natural 
recording systems to carry the record of climate back into the past. 
Records from well-dated tree rings, annually layered (or varved) lake 
sediments, and ice cores resemble the historical data in that values 
can be associated with individual years and may be calibrated with 
modern data to extend the climatic record for many centuries, and in 
certain favored sites for as long as 8000 to 10,000 years. Other record- 
ing systems, such as the pollen concentration in lake sediments and 
fossil organisms and oxygen isotopes in ocean sediments, have less 
resolution but may provide continuous records extending over many 
tens of thousands of years. These and other characteristics of proxy 
climatic data sources are summarized in Table A.l. 

In general, the older geological records provide only fragmentary 
and generally qualitative information but constitute our only records 
extending back many millions of years. For the past one million years, 
however, and especially for the past 100,000 years, the record is rela- 
tively continuous and can be made to yield quantitative estimates of 
the values of a number of significant climatic parameters. These in- 
clude the total volume of glacial ice (and its inverse, the sea-level), 
the air temperature and precipitation over land, the sea-surface 
temperature and salinity for much of the world ocean, and the general 
trend of air temperature over the polar ice caps. 

Like sensing systems made by man, each natural paleoclimatic 
indicator must be calibrated, and each has distinctive performance 
characteristics that must be understood if the data are to be interpreted 
correctly. In discussing these sources it is useful to distinguish between 
those paleoclimatic indicators that are more or less continuous re- 
corders of climate, such as tree rings and varves, and those whose 
records are episodic, such as mountain glaciers. We should also con- 
sider the minimum attainable sampling interval that is characteristic of 
a particular paleoclimatic indicator (see Table A.l). Thus, tree rings, 
varves, and some ice cores can be sampled at intervals of one year, 
pollen or other sedimentary fossil samples only rarely represent less 
than about 100 years, and many geological series are sampled over 
intervals representing a thousand years or more. These figures reflect 
differences in the resolving power of each proxy indicator. Climate- 
induced changes in a plant community as reflected in pollen concentra- 
tions, for example, are relatively slow; the high-frequency information is 
lost, but low-frequency changes are preserved. In contrast, tree-ring 



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134 UNDERSTANDING CLIMATIC CHANGE 

records and isotopic records in ice cores respond yearly and even 
seasonally in favored sites. 

Each proxy record also has a characteristic chronologic and geo- 
graphic range over which it can be used effectively. Tree-ring records 
go back several thousand years at a number of widely distributed con- 
tinental sites, pollen records have the potential of providing synoptic 
coverage over the continents for the past 12,000 years or so, and a 
nearly complete record of the fluctuating margins of the continental 
ice sheets is available for about the last 40,000 years. Planktonic and 
benthic fossils from deep-sea cores can in principle provide nearly 
global coverage of the ocean going back tens of millions of years, al- 
though sampling difficulties have thus far limited our access to sediments 
deposited during the last several hundred thousand years. 

Although instrumental records best provide the framework neces- 
sary for the quantitative understanding of the physical mechanisms of 
climate and climatic variation with the aid of dynamical models, the 
increasingly quantitative and synoptic nature of paleoclimatic data 
will add a much-needed perspective. As discussed elsewhere in this 
report, it is therefore important that the historical and proxy records 
of past climate be systematically assembled and analyzed, in order to 
provide the data necessary for a satisfactory description of the earth's 
climates. 



Instrumental and Historical Methods of Climate Reconstruction 

Over the past three centuries, the development of meteorological 
instruments and appropriate "platforms" for sensing the state of the 
atmosphere-hydrosphere-cryosphere system has produced an important 
storehouse of quantitative information pertaining to the earth's climates. 
Time series of these records show that climate undergoes considerable 
variation from year to year, decade to decade, and century to century. 
From a practical viewpoint, much of this information mirrors the 
economically more important climatic variations, as found, for example, 
in the changes of crop and animal production, the patterns of natural 
flora and fauna, and the variations of the levels of lakes and streams 
and the extent of ice. Generally the term "climate" is understood to 
describe in some fashion the "average" of such variations. As discussed 
in Chapter 3, a complete description of a climatic state would also 
include the variance and extremes of atmospheric behavior, as well as 
the values of all parameters and boundary conditions regarded as ex- 
ternal to the climatic system. 

In discussing the reconstruction of climates from instrumental data, 
several characteristics of past and present observational systems should 



APPENDIX A 135 

be considered. First, instrumental observations have been obtained for 
the most part for the purpose of describing and forecasting the weather. 
Hence, although extensive records of such weather elements as tempera- 
ture, precipitation, cloudiness, wind, and observations are available, 
they are inadequate for many climatic purposes. There exist few direct 
measurements related to the thermal forcing functions of the at- 
mosphere-hydrosphere-cryosphere system, such as the solar constant; 
the radiation, heat, and moisture budgets over land and ocean surfaces; 
the vegetative cover; the distribution of snow and ice; the thermal 
structure of the oceanic surface layer; and atmospheric composition 
and turbidity. 

Second, observational records may be expected to contain errors due 
to changes in instrument design and calibration and to changes in 
instrument exposure and location. There is therefore a need to establish 
and maintain conventional observations at reference climatological 
stations and a need to identify, insofar as possible, "benchmark" records 
of past climate. Such observations are needed to supplement the climatic 
monitoring program described elsewhere (see Chapter 6). 

Third, the time interval over which portions of the climatic system 
need to be described are very different. If, for example, the fluctuations 
in the volume and extent of the polar ice caps are to be studied, a 
time interval of order 100,000 years (the maximum residence time of 
water in the ice caps) is required. Or if atmospheric interaction with 
the deep oceans is to be considered, then a time interval of the order 
500 years (the residence time of bottom water) is required. It is there- 
fore apparent that the period of instrumental records covering the past 
century or two is long enough only to have sampled a portion of such 
climatic responses, and that our information on older climates must 
come from historical sources and from the various natural (proxy) 
indicators of climate described earlier. Although such records will al- 
ways be fragmentary, we should recognize their unique value in describ- 
ing the past behavior of the earth's climatic system. 

For practical reasons, it has been convenient to compute climatic 
statistics over relatively short intervals of time, such as 10, 20, or 
30 years, and to designate the 30-year statistics as climatic "normals." 
It is important to note, however, that the most widely accepted climatic 
"normals" (for the period 1931-1960) represent one of the most ab- 
normal 30-year periods in the last thousand years (Bryson and Hare, 
1973). As noted elsewhere in this report, the entire last 10,000 years are 
themselves also abnormal in the sense that such (interglacial) climates 
are typical of only about one tenth of the climatic record of the last 
million years. 

While continuous observations of atmospheric pressure, temperature, 



136 UNDERSTANDING CLIMATIC CHANGE 

and precipitation are available at a few locations from the late seven- 
teenth century, such as the record of temperature in Central England 
assembled by Manley (1959), it is only since the early part of the 
eighteenth century that the spatial coverage of observing stations has 
permitted the mapping of climatic variables on even a limited regional 
scale. These and other scattered early observations of rainfall, wind 
direction, and sea-surface temperature have been summarized by Lamb 
(1969). Only since about 1850 are reliable decadal averages of surface 
pressure available for most of Europe, and only since about 1900 are 
there reliable analyses for the midlatitudes of the northern hemisphere, 
as shown in Figure A. 3. And only since about 1950 does the surface 
observational network begin to approach adequate coverage over the 
continents; large portions of the oceans, particularly in the southern 
hemisphere, remain inadequately observed. 

For the climate of the free atmosphere, the international radiosonde 
network permits reliable analyses for the midlatitudes of the northern 
hemisphere only since the 1950's, and less than adequate coverage 
exists over the rest of the globe. Beginning in the 1960's, routine ob- 
servations from satellite platforms have begun to make possible global 
observations of a number of climatic variables, such as cloudiness, the 
planetary albedo, and the planetary heat budget. Yet many important 
quantities, such as the heat and moisture budgets at the earth's surface 
and the thermal structure and motions of the oceanic surface layer, 
remain largely unobserved on even a local scale. 

Biological and Geological Methods of Climate Reconstruction 

During the first three decades of the nineteenth century, Venetz in 
Switzerland and Esmark in Norway inferred the existence of a pre- 
historic ice age from the study of vegetation-covered moraines and other 
glacial features in the lower reaches of mountain valleys. After a century 
of effort, the literature of paleoclimatology has become so diverse, and 
so burdened by stratigraphic terminology, that it is useful to provide a 
summary of paleoclimatic techniques. 

The quantitative description of past climates as determined by bio- 
logical and geological records requires the development of paleoclimatic 
monitoring techniques and the construction of time scales by suitable 
chronometric or dating methods. In general, the second of these prob- 
lems is the more difficult. 

Beyond the range of 14 C dating (the past 40,000 years), it is only 
since about 1970 that the main chronology of the climate of the past 
100,000 years has become clear; and only since 1973 that the main 
features of the chronology of the past million years have been estab- 



APPENDIX A 



137 




• a 



mm ! i 




80" 0' 20* 



100* 140* 

S>. ! !■ 








FIGURE A.3 Growth of the network of surface pressure observa- 
tions and of the area that can be covered by reliable 10-year 
average isobars (Lamb, 1969). (a) 1750-1759, (b) 1850-1859, (c) 
1950-1959. 



138 UNDERSTANDING CLIMATIC CHANGE 

lished. Key discoveries in these time ranges have been in the sea-level 
records of oceanic islands and in the sedimentary records of deep-sea 
cores. In preparing this survey, the chronology of these records has been 
used as a framework into which the data from more fragmentary or 
poorly dated records have been fitted. 

Monitoring Techniques 

The problem of developing a paleoclimatic monitoring technique — 
or finding something meaningful to plot — may be broken down into 
three subproblems. A natural climatic record must be (a) identified, 
(b) calibrated, and (c) obtained from a stable recording medium. 

Identification of Natural Climatic Records A number of different 
monitoring techniques that can provide data for paleoclimatic inference 
are summarized in Table A.l and are based on observations of fossil 
pollen, ancient soil types, lake deposits, marine shore lines, deep-sea 
sediments, tree rings, and ice sheets and mountain glaciers. The tech- 
niques that are emphasized here are those that in general yield more or 
less continuous time series. Other types of proxy data are also useful 
in the reconstruction of climatic history (see, for example, Flint, 1971, 
or Washburn, 1973). 

Calibration of Paleoclimatic Records Many proxy records must be 
calibrated to provide an estimate of the climatic parameter of interest. 
The elevation of an ancient coral reef, for example, is a record of a 
previous sea level; but before it can be used for paleoclimatic purposes 
the effect of local crustal uplift or subsidence must be removed (Bloom, 
1971; Matthews, 1973; Walcott, 1972). 

Another example may be cited from paleontology, where the taxo- 
nomic composition of fossil assemblages and the width of tree rings 
are known to reflect the joint influence of several ecological and en- 
vironmental factors of climatic interest. Here appropriate statistical 
techniques are used to define indices that give estimates of the individual 
paleoclimatic parameters, such as air temperature, rainfall, or sea- 
surface temperature and salinity. In the case of tree rings, although 
each tree responds only to the local temperature, moisture, and sun- 
light, for example, by averaging over many sites, the trees' response may 
be related to the large-scale distribution of rainfall and surface tempera- 
ture. In this way a statistical relationship may be established with a 
variety of parameters, even though they may not be direct causes of 
tree growth. When such tree-ring data are carefully dated they can 



APPENDIX A 139 

thus provide estimates of the past regional variations of climatic elements 
such as precipitation, temperature, pressure, drought, and stream flow 
(Fritts et al, 1971). These methods yield what are called transfer func- 
tions, which serve to transform one set of time-varying signals to another 
set that represents the desired paleoclimatic estimates. In addition to 
their application to tree-ring data, multivariate statistical-analysis 
techniques have been successfully applied to marine fossil data (Imbrie 
and Kipp, 1971; Imbrie, 1972; Imbrie et al., 1973) and to fossil pollen 
data (Bryson et al., 1970; Webb and Bryson, 1972). Typical results 
indicate, for example, that average winter sea-surface temperatures 
18,000 years ago in the Caribbean were about 3°C lower than today, 
while those in midlatitudes of the North Atlantic were about 10°C 
below present levels. 

The oxygen isotope ratio 18 0/ 16 as it is preserved in different ma- 
terials is used in three separate paleoclimatic monitoring techniques. 
Although the results are interpreted differently, in each technique the 
ratio is measured as the departure 8 18 from a standard, with positive 
values indicating an excess of the heavy isotope. One technique ex- 
amines the ratio in polar ice caps, where the values of 8 18 are generally 
on the order of 30 parts per thousand lower than in the oceanic reservoir, 
because of the precipitation and isotopic enrichment that accompanies 
the transport of water vapor into high latitudes. As shown by Dans- 
gaard (1954) and by Dansgaard et al. (1971) the value of 8 ls O in each 
accumulating layer of ice is closely related to the temperature at which 
precipitation occurs over the ice. Although complicating effects make it 
impossible to convert the 8 ls O curve into an absolute measure of air 
temperature, the isotopic time series are extraordinarily detailed. 

Another isotopic technique records 8 ls O in the carbonate skeletons 
of planktonic marine fossils (Emiliani, 1955, 1968). Here the ratio is 
determined by the isotopic ratio and temperature in the near-surface 
water in which the organisms live. Work by Shackleton and Opdyke 
(1973) demonstrates that the observed ratio is predominantly influ- 
enced by the isotopic ratio in the seawater. Hence the isotopic curve 
reflects primarily the changing volume of polar ice, which, upon melting, 
releases isotopically light water into the ocean. 

A third technique measures the isotopic ratio in benthic fossils whose 
skeletons reflect conditions prevailing in bottom waters. By making 
the assumption that the temperature of bottom water underwent little 
change over the past million years, the difference between the isotopic 
ratio observed in benthic and planktonic fossils can be used to estimate 
changes in surface-water temperatures. Initial application of this tech- 
nique (Shackleton and Opdyke, 1973) provides an independent con- 



140 UNDERSTANDING CLIMATIC CHANGE 

firmation of the previously cited estimate of glacial-age Caribbean 
temperatures obtained by paleontological techniques. Over time spans 
on the order of tens of millions of years, measurements of 8 ls O in 
benthic fossils offer a means of tracing changes in bottom water in 
which the effects of changing polar temperatures and ice volumes are 
combined (Douglas and Savin, 1973). 

Evaluation of the Recording Medium All paleoclimatic techniques 
require that ambient values of a climatic parameter be preserved within 
individual layers of a slowly accumulating natural deposit. Such de- 
posits include sediments left by melting glaciers on land; sediments 
accumulating in peat bogs, lakes, and on the ocean bottom; soil layers; 
layers accumulating in polar ice caps; and the annual layers of wood 
formed in growing trees. Ideally, a recording site selected for paleo- 
climatic work should yield long, continuous, and evenly spaced time 
series. The degree to which these qualities are realized varies from 
site to site, so that distortions and nonuniformities in each record must 
be identified and removed. The stratigraphic techniques by which this 
screening is accomplished will not be discussed here, although the reader 
should be aware that (with the exception of tree rings) some degree of 
chronological distortion will occur in all paleoclimatic curves where 
chronometric control is lacking. 

To enable the reader to form his own judgments as to the chronology 
of past climatic changes, most of the paleoclimatic curves given in this 
report show explicitly the time control points between which the data 
are spaced in proportion to their relative position in the original sedi- 
mentary record. This procedure assumes that accumulation was con- 
stant between the time controls, which is a reasonable assumption in 
favorable environments. In other cases this assumption introduces a 
distortion in the signal and a consequent uncertainty in the timing of 
the inferred climatic variations. 

Each of the recording media used in paleoclimatography has char- 
acteristic limitations and advantages. As summarized in Table A.l, 
the reconstruction of past climates requires evidence from a variety of 
techniques, each yielding time series of different lengths and sampling 
intervals and reflecting variations in different regions. The tree-ring 
record, for example, provides evenly spaced and continuous annual 
records, but only for the past few thousand years. The ice-margin record 
of both valley and continental glaciers is discontinuous, especially 
prior to about 20,000 years ago, because each major glacial advance 
tends to obliterate (or at least to conceal) the earlier evidence. Records 
of lake levels and sea levels are also discontinuous. The former rarely 



APPENDIX A 141 

extend back more than 50,000 years, although the latter extend back 
several hundred thousand years. Soil sequences display great variability 
in sedimentation rate but provide continuous climatic information for 
sites on the continents where other records are not available (or are 
discontinuous); in favored sites, the soil record extends back about a 
million years. Pollen records are usually continuous but are rarely 
longer than 12,000 years. Deep-sea cores provide material for the 
study of fossils, oxygen isotopes, and sedimentary chemistry. These 
records are relatively continuous over the past several hundred thousand 
years and are distributed over large parts of the world ocean. Their 
relatively uniform but low deposition rates, however, generally limit the 
chronological detail obtainable. Cores taken in the continental ice 
sheets provide a detailed and generally continuous record for many 
thousands of years, although their interpretation is handicapped by the 
lack of fully adequate models of the ice flow with its characteristic 
velocity-temperature feedback. 



Chronometric Techniques 

The problem of constructing a paleoclimatic chronology has been ap- 
proached by four direct methods and one indirect method. 

Dendrochronology The most accurate direct dating is achieved in 
tree-ring analysis, in which many records with overlapping sets of rings 
are matched. With sufficient samples, virtual certainty in the dates 
of each annual layer may be obtained, and a year-by-year chronology 
can be established for periods covered by the growth records of both 
living and fossil trees. Such records are especially valuable for studying 
variations of climate during the last few hundred years and can be 
extended to many of the land regions of the world. 

Analysis of Annually Layered Sediments In favored locations, lakes 
with annually layered bottom sediments provide nearly the same time 
control as do tree rings. Some ice cores and certain marine sediment 
cores from regions of high deposition rates also contain distinct annual 
layers. These data, along with tree rings and historical records, are the 
only source of information on the high-frequency portion of the 
spectrum of climatic variation. 

Radiocarbon Dating The advent of the 14 C method in the early 1950's 
was a major breakthrough in paleoclimatography, for it made possible 
the development of a reasonably accurate absolute chronology of the 



; 






142 UNDERSTANDING CLIMATIC CHANGE 

past 40,000 years in widely distributed regions. Prior knowledge was 
essentially limited to dated tree-ring sequences (for the past several 
thousand years) and to varve-counted sequences in Scandinavia (ex- 
tending back to about 12,000 years). The 14 C method has an accuracy 
of about ±5 percent of the age being determined; that is, material 
10,000 years old could be dated within the range 9500-10,500 years. 
The calibration of ll C ages against those determined from dendro- 
chronology gives insight into the variations of atmospheric 14 C produc- 
tion rates over the past 7000 years (Suess, 1970). 

Decay of Long-Lived Radioactivities These methods employ daughter 
products of uranium decay or the production of 10 Ar through potassium 
decay. Used under favorable circumstances, one of the uranium methods 
(the decay of 230 Th) can provide approximate average sedimentation 
rates in deep-sea cores. The other method (the growth of 230 Th) can 
be used successfully on fossil corals to provide discrete dates for shore- 
line features recording ancient sea levels. Together, these techniques 
have provided a reasonably satisfactory chronology of the past 200,000 
years with a dating accuracy of about ±10 percent. Our chronology 
for older climatic records is based on the well-known K/Ar technique, 
applied to terrestrial lava flows and ash beds. This technique has pro- 
vided, for example, the important dates for paleomagnetic reversal 
boundaries. 

Stratigraphic Correlation with Dated Sequences Much of the absolute 
chronology of climatic sequences is supplied by an indirect method, 
namely, the stratigraphic correlation of specific levels in an undated 
sequence with dated sequences from another location. For example, a 
particular glacial moraine that lacks material for 14 C dating may be 
identified with another formed at the same time that has datable ma- 
terial. Such correlation by direct physical means is limited to relatively 
small regions, however, and stratigraphic correlation techniques must 
be used. Three such methods form the backbone of the chronology 
of paleoclimate : biostratigraphy, isotope stratigraphy, and paleomag- 
netic stratigraphy. 

The techniques of biostratigraphy use the levels of extinction or 
origin of selected species as the basis for correlation. This method has 
enabled Berggren (1972), for example, to devise a time scale of the 
past 65,000,000 years that is widely used as a basis for historical inter- 
pretation. Isotope stratigraphy, applicable only to the marine realm, 
makes use of the fact that the record of oxygen isotope variations — 



APPENDIX A 143 

which reflects chiefly the global ice volume — has distinctive char- 
acteristics that permit the correlation of previously undated sequences. 
The application of paleomagnetic correlation techniques has revolu- 
tionized our approach to the climatic history of the past several million 
years. Their importance stems from the fact that the principal magnetic 
reversal boundaries, which have occurred irregularly about every 
400,000 years, are recorded in both marine and continental sedimentary 
sequences. 

Regularities in Climatic Series 

On the assumption that climatic changes are more than just random 
fluctuations, paleoclimatologists have long sought evidence of regu- 
larities in proxy records of the earth's climatic history. Many have 
found what they believe to be firm evidence of order and refer to the 
chronological patterns as "cycles." Although the number of records 
is limited, and hard statistical evidence is sometimes lacking, it is never- 
theless convenient to describe some of the larger climatic changes in 
terms of quasi-periodic fluctuations or cycles with specified mean wave- 
lengths or periods, in the sense that they describe the apparent repetitive 
tendency of certain sequences of climatic events. For example, many 
aspects of the global ice fluctuations during the last 700,000 years 
may be summarized in terms of a 100,000-year cycle [see Figure 
A.2(e)]. Each such period is marked by a gradual transition from a 
relatively ice-free climate (or interglacial) to a short, intense glacial 
maxima and followed by an abrupt return to ice-free climate. No two 
such cycles are the same in detail, however, and should not be construed 
as indicating strict periodicities in climate. 

Some paleoclimatic cycles may be periodic, or at least quasi-periodic, 
and rest on evidence that is exclusively or mainly chronological. The 
best example is the approximate 100,000-year cycle found from the 
spectral analysis of time series, such as that shown in Figure A. 4. For 
the 100,000-year cycle, as well as some of the higher-frequency fluctua- 
tions that modify it, there is circumstantial evidence to suggest that 
these have in some way been induced by secular variations of the 
earth's orbital parameters, which are known to alter the latitudinal pat- 
tern of the seasonal and annual solar radiation received at the top of 
the atmosphere. For the 2500-year (and shorter) fluctuations suggested 
by some proxy data series, the causal mechanism is unknown. 

With the possible exception of the approximately 100,000-year quasi- 
periodic fluctuation referred to above, the quasi-biennial oscillation (of 



144 



UNDERSTANDING CLIMATIC CHANGE 



PERIOD IN THOUSANDS OF YEARS 

100 50 33 25 20 17 14 12 11 

T 



10 



. 




CYCLES PER 100,000 YEARS 

FIGURE A.4 Power spectrum of climatic fluctuations during the last 
600,000 years according to Imbrie and Shackleton (1974). The data 
analyzed are time-series observations of 5 18 in fossil plankton in 
the upper portion of a deep-sea core in the equatorial Pacific, inter- 
polated at intervals of 2500 years (Shackleton and Opdyke, 1973). 
This ratio reflects fluctuations in global ice-volume. 



L 



APPENDIX A 145 

2-3 year period) is the only quasi-periodic oscillation whose statistical 
significance has been clearly demonstrated. This is not to say that other 
such fluctuations in climate are absent but rather that much further 
analysis of proxy records is required. A question of equal importance 
is the shape of the continuum variance spectrum of climatic fluctua- 
tions. A uniform distribution of variance as a function of frequency 
(or "white noise") would imply a lack of predictability in the statistical 
sense or a lack of "memory" of prior climatic states. A "red-noise" 
spectrum, on the other hand, in which the variance decreases with in- 
creasing frequency, implies some predictability in the sense that suc- 
cessive climatic states are correlated. The existence of nonzero auto- 
correlations in such a spectrum implies that some portion of the climatic 
system retains a "memory" of prior states. In view of the relatively 
short memory of the atmosphere, it seems likely that this is provided by 
the oceans on time scales of years to centuries and by the world's major 
ice sheets on longer times scales. 

An initial estimate of the variance spectrum of temperature has been 
made from the fluctuations on time scales from 1 to 10,000 years by 
Kutzbach and Bryson (1974) and is shown in Figure A. 5. This spectrum 
has been constructed from a combination of calibrated botanical, chemi- 
cal, and historical records, along with instrumental records in the North 
Atlantic sector. As may be seen in Figure A.5(a), the variance spectral 
density increases with decreasing frequency (increasing period) over 
the entire frequency domain but is most pronounced for periods longer 
than about 30 years. In Figure A.5(b), the spectrum of the same time 
series is shown with frequency on a logarithmic scale and the ordinate 
as spectral density (V) times frequency (/), so that equal areas repre- 
sent equal variance. Again, for periods longer than about 30-50 years, 
the observed temperature spectrum is seen to depart significantly from 
the white-noise continuum associated with the high-frequency portion of 
the spectrum. The determination of the character of the variance spec- 
trum of the various climatic elements remains largely a task for the 
future. 

We will use the term "cycle" in the following paragraphs to designate 
such quasi-periodic sequences of climatic events, since there appears 
to be no other word or phrase that conveys the concept of a series of 
generally similar events spaced at reasonably regular intervals in time. 
Although our knowledge of the record of past climates has improved 
greatly during the last decade, a much broader paleoclimatic data base 
is clearly required. Only then can adequate spectral analyses be per- 
formed and the spatial and temporal structure of paleoclimatic variations 
firmly established. 







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148 



UNDERSTANDING CLIMATIC CHANGE 



CHRONOLOGY OF GLOBAL CLIMATE 

Period of Instrumental Observations 

A variety of meteorological indices have been used to characterize the 
climate and its temporal variations during the past century or more 
of extensive observations. Global- or hemisphere-averaged indices such 
as the surface temperature index shown in Figure A. 6 are often used for 
this purpose. This index clearly suggests a worldwide warming begin- 
ning in the 1880's, followed by a cooling since the 1940's. The warming 
may be recognized as the last part of a complex but recognizable trend 
that has persisted since the end of the seventeenth century [see 
Figure A.2(b)]. 

The geographic patterns of temperature change during these overall 
warming and cooling epochs show considerable variability, with the 
largest changes concentrated in the polar regions of the northern 
hemisphere. Mitchell (1963) has shown that the pattern of temperature 
change during recent decades is consistent with concomitant changes in 
the large-scale atmospheric circulation as reflected in sea-level pressure. 
Less attention has been given to the more complex relationships be- 
tween circulation variation and changing precipitation patterns, although 
Kraus (1955a, 1955b) and Lamb (1969) have considered this aspect 
of the problem. 

Lamb and Johnson (1959, 1961, 1966) and Lamb (1969) have 
made an extensive analysis of certain features of atmospheric circulation 




I960 



FIGURE A.6 Recorded changes of annual mean temperature of the 
northern hemisphere as given by Budyko (1969) and as updated after 
1959 by H. Asakura of the Japan Meteorological Agency (unpublished 
results). 



APPENDIX A 149 

based on the observed and historically reconstructed surface pressure 
maps for individual months since about 1750. They have extracted such 
indices as the strength of the zonal and meridional flow, the position 
and wavelength of trough-ridge patterns, and the position and strength 
of subtropical pressure systems. The year-to-year and decade-to-decade 
changes in these indices reflect changing large-scale circulation pat- 
terns, which in turn are associated with changing patterns of tempera- 
ture and precipitation. From the instrumental era for the North Atlantic 
sector, the typical variability over 20- to 30-year intervals of the low- 
level westerlies is ±1-2 m sec- 1 , and that for the planetary-scale 
circulation features (such as large-scale troughs and ridges) is ±1-2 
deg latitude and ± 10-20 deg longitude. 

Although changes in the position, pattern, and intensity of the gen- 
eral circulation are interrelated, such empirical studies suggest that 
longitudinal shifts have the most significant effects on the climatological 
temperature and precipitation patterns, at least for middle and higher 
latitudes. Examples of such shifts are shown in Figure A. 7. In tropical 
and subtropical latitudes, on the other hand, latitudinal shifts appear 
to be more closely related to regional climatic variations, as indicated 
by the data of Figure A. 8. 

The Last 1000 Years 

To obtain an indication of the climate in the northern hemisphere for 
the last 1000 years, Lamb (1969) has compiled manuscript references 
on the character of European weather and has developed an index of 
winter severity, as shown in Figure A.2(b) and A. 9. Although different 
longitudes show somewhat different results, the trends shown by this 
index (the excess number of unusually mild or unusually cold winter 
months over months of opposite character) for the period since about 
1700 have been validated by comparison with thermometer records. 
Other portions of the record have been cross-checked with data on 
glacial fluctuations, oxygen isotope variations, and tree growth, so that 
the main characteristics of European climate during this period are 
reasonably well known. LaMarche (1974) has constructed temperature 
and moisture records from the ring-width variations in trees at high- 
altitude arid sites in California [see Figure A.9(c)]. Comparisons of his 
data with those from Europe shown in Figure A.9(d) indicate a degree 
of synchrony in the major fluctuations of temperature between the west 
coast of North America and Western Europe during the last 1000 years. 
The early part of the last millenium (about a.d. 1100 to 1400) is 
sometimes called the Middle Ages warm epoch but was evidently not as 



150 



UNDERSTANDING CLIMATIC CHANGE 



1790-1829 r 

1800-1839 

1810-1849 

1820-1859 |- 

1830-1869 

1840-1879 - 

1850-1889 - 

1860-1899 

1870-1909 

1880-1919 

1890-1929 

1900-1939 

1910-1949 

1920-1959 hr 



Trough 




60 °W 



20 U E 



1780-1819 



1800-1839 



1820-1859 



1840-1879 ~ 



1860-1899 - 



1880-1919 - 



1900-1939 - 



1920-1959 



Trough 



Ridge 



J II I L 



70 W 60 



50 



40 30 20 10 10 °E 



FIGURE A.7 Forty-year running means of the longitudes of the semipermanent 
surface pressure troughs and ridges in the North Atlantic (Lamb, 1969). (a) at 
45 °N in January; (b) at 55 °N in July. 



APPENDIX A 



151 



2-3 




120. 



II 100- 



80 



120 



ll 

s 100- 



(a) 



- (b) 



i i ! I I i I 1 ' ' F ' ' I ' I ' ' I 



« I 1 1 1 ii 1 r— » 1 r 1 — 

1900 10 20 30 40 50 60 70 

YEAR 

FIGURE A.8 Twenty-year running means of selected climatic indices (Winstanley, 1973). 
(a) Frequency (days per year) of westerly weather type over the British Isles (from Lamb, 
1969); (b) winter-spring rainfall (mm) at 14 stations in North Africa and the Middle East; 
(c) summer monsoon rainfall (mm) at eight stations in the Sahel of North Africa and 
northwest India. 



warm as the first half of the twentieth century. The period from about 
1430 to 1850 is commonly known as the Little Ice Age, and some 
records indicate that this period had cold maxima in the fifteenth and 
seventeenth centuries. From such evidence we infer that the atmospheric 
circulation may have been more meridional than at present and char- 
acterized in western Europe and western North America by short, wet 
summers and long, severe winters. 

During the Little Ice Age many glaciers in Alaska, Scandinavia, and 
the Alps advanced close to their maximum positions since the last 
major ice age thousands of years ago. A visual impression of these 
events in the French Alps was shown in Figure A.l. The expansion of 
the Arctic pack ice into North Atlantic waters caused the Norse colony 
in southwest Greenland to become isolated and perish; and in Iceland, 
grain that had grown for centuries could no longer survive. 



152 



UNDERSTANDING CLIMATIC CHANGE 



PARIS - LONDON GREENLAND ICE CORE 

HISTORIC 8 O 18 (O/OO) 

WINTER SEVERITY 
COLD WARM -30 -29 -2 ~ 



1900 



1700 



1500 



1300- 




TREE 

RING WIDTH 

MM 





I .3 4 .5 .6 .7 


1900 




1700 


^CT WHITE 
- <T MTNS. 


1500 




< 

UJ 1300 


<=Samarche 


1100 




900 





CHANGE IN 
MEAN ANNUAL 
TEMPERATURE 

00 0.5 1.0 1.5 „ 



- 2001 



CENTRAL 
ENGLAND] 400 



LAMB 



600 m 



800 



1000 



FIGURE A.9 Climatic records of the 
past 1000 years, (a) The 50-year moving 
average of a relative index of winter 
severity compiled for each decade from 
documentary records in the region of 
Paris and London (Lamb, 1969). (b) A 
record of 5 18 values preserved in the 
ice core taken from Camp Century, 
Greenland (Dansgaard et al., 1971). (c) 
Records of 20-year mean tree growth 
at the upper treeline of bristlecone 
pines, White Mountains, California (La- 
Marche, 1974). At these sites tree 
growth is limited by temperature with 
low growth reflecting low temperature, 
(d) The 50-year means of observed and 
estimated annual temperatures over 
central England (Lamb, 1966). 



The Last 5000 Years 

As indicated in Figure A.2(c), the period from 7000 to 5000 years ago 
was marked by temperatures warmer than those that prevail today [and 
is thus sometimes known as the hypsithermal interval (Flint, 1971)]. 
The last 5000 years is characterized by generally declining temperatures 
and a trend toward more extensive mountain glaciation (but not ice 
sheets) in all parts of the world (Porter and Denton, 1967). Close 



APPENDIX A 



153 




TREE GROWTH 
FLUCTUATIONS AT 
UPPER TREELINE 



HOLOCENE 
GLACIER 
FLUCTUATIONS 
(Denton a Karien,l973) 



3000 



2000 



1000 



B.C. A.D. 



1000 



FIGURE A.10 Climatic records of the past 5000 years, (a) Average (100-year mean) ring 
widths of bristlecone pine at the upper treeline in the White Mountains of California 
(LaMarche, 1974). Positive growth departures indicate warm-season (April-October) 
temperatures above the long-term mean, with a total temperature range of about 4°F. 
(b) Records of the advance and retreat of Holocene Alaskan glaciers (Denton and 
Karlen, 1973). 



examination of the records of mountain glaciers, treelines, and tree 
rings suggests that this general cooling trend was itself punctuated in 
many parts of the world by cold intervals centered at about 5300, 2800, 
and 350 years ago, as shown in Figure A. 10. Much further analysis 
of proxy climatic records during this period is needed, including the 
evidence available from historical sources. 



The Last 25,000 Years 

The climatic record of the last 25,000 years is largely concerned with 
the present interglacial interval (or Holocene) and the terminal phases 
of the last major glaciation [see Figure A. 2(d)]. Although the maxi- 
mum ice extent occurred between about 22,000 and 14,000 years ago 
(see Figure A.ll) the curves of ice accumulation and decline are not 
identical for the various ice sheets. The Laurentide ice sheet (which 
covered parts of eastern North America) and the Scandinavian ice 
sheet (which covered parts of northern Europe) reached their maximum 
extent between 22,000 and 1 8,000 years ago, while the Cordilleran ice 
sheet achieved its maximum only 14,000 years ago. The maximum areas 
of the northern hemisphere ice sheets during the past 25,000 years 
were about 90 percent of the maxima during the last million years of 
the Pleistocene (see Table A.2). 

Widespread deglaciation began rather abruptly about 14,000 years 



154 



UNDERSTANDING CLIMATIC CHANGE 



VARIATIONS IN 
CARIBBEAN 
PLANKTON 
(Core VI2-I22) 

Faunal Index Tw 
gg 25 



FLUCTUATION IN THE MARGINS OF 
NORTHERN HEMISPHERE ICE SHEETS 



THREE 



(Erie Lobe of Lauren tide ice sheet) 
2000 1500 1000 500 

I I I I I 



(Cordilleron Ice Sheet in Fraser-Puget 
Lowland) 
i I I i I L_J 



(Eastern Sector of Scandinavian Ice Sheet 
i i i i i_ 




DISTANCE (km) FROM CENTER OF OUTFLOW 

• =C 14 dates in VI2-I22 

(ice margin fluctuation chronology controlled by numerous C 14 data) 



FIGURE A.ll Climatic records of the past 40,000 years, (a) Fluctuations in Caribbean 
plankton (core V12-122) interpreted as a record of sea-surface temperature in C. (b) 
Fluctuations in the isotopic composition of Caribbean plankton interpreted as a record of 
changing global ice-volume. Both records are from Imbrie et al. (1973). Curves (c), (d), 
and (e) are time-distance plots of changes in the margins of three northern hemisphere 
ice sheets. Curve (c) is from Dreimanis and Karrow (1972), curves (d) and (e) are due to 
G. H. Denton, University of Maine (unpublished). The chronology of curves (a) and (b) 
is controlled by 14 C dates shown by solid circles; the ice-margin curves are controlled 
by numerous a4 C dates. 



ago, and the waning phases of the continental ice sheets were char- 
acterized by substantial marginal fluctuations (Dreimanis and Karrow, 
1972), as shown in Figure A.ll. The Cordilleran ice sheet, which had 
just attained its maximum extent, melted rapidly and was gone by 
10,000 years ago. The Scandinavian ice sheet lasted only slightly 
longer and retreated at the rate of about 1 km per year between about 
10,000 and 9000 years ago. The climatic instability suggested by these 
fluctuations in the margins of the northern hemisphere's major ice sheets 
is corroborated by the records from fossil pollen, deep-sea cores, ice 
cores, and sea-level variations, as shown in Figure A. 12, and by 
lacustrine records in western North America and Africa. By 8500 years 
ago the ice conditions in Europe had reached essentially their present 



APPENDIX A 



155 



TABLE A.2 Characteristics of Existing Ice Sheets and of the Maximum 
Quaternary Ice Cover " 







Area 






(10 12 m 2 ) 


Existing Glaciers 






Greenland 




1.80 


Spitsbergen + Iceland 




0.07 


Canadian Archipelago 




0.15 


North America 




0.08 


Europe + Asia 




0.17 


South America 




0.03 


Antarctica 


rAL AREA 


12.59 


TOT 


14.99 (3% of earth's surface) 


TOTAL ICE 


VOLUME 6 


2.5 x 10 7 km 3 


EQUIVALENT SEA-LEVEL 


CHANGE 


70 m 


Maximum Quaternary Glaciation 




Greenland 




2.30 


Spitsbergen + Iceland 




0.44 


Alaska 




1.03 


Cordillera 




1.58 


Laurentide 




13.39 


Scandinavia 




6.67 


Europe 




0.09 


Asia 




3.95 


South America 




0.87 


Antarctica 




13.81 


Other 


AL AREA 


0.04 


TOT 


44.17 (9% of earth's surface) 


TOTAL ICE 


VOLUME 6 


7.5xl0 7 km 3 


EQUIVALENT SEA-LEVEL 


CHANGE 


210 m 



"From Flint (1971). 

6 Based on the present ice thickness of 1700 m in Greenland and Antarctica. 



state, and in North America the ice sheets had shrunk to about their 
present extent by about 7000 years ago. 

How widespread and synchronous these fluctuations were is not yet 
known, but evidence is growing that there were several periods of 
widespread cooling and glacial expansion in the regions bordering the 
Atlantic Ocean [see Figure A. 2(c)], spaced about 2500 years apart. 
One of these glacial advances (the Younger Dryas event, about 10,800 
to 10,100 years ago) was a climatic event of unparalleled abruptness 
in Europe, establishing itself within a century or less and lasting for 
some 700 years. Northern forests that had advanced during the pre- 



156 



UNDERSTANDING CLIMATIC CHANGE 



U 

CO 



MINNESOTA 
POLLEN CORE 
63 66 69 



NORTH ATLANTIC 
CORE V-23-81 

? 8 '? 



GREENLAND 
ICE CORE 



DATED SHORE- 
LINE FEATURES 




38 -33 -28 25 50 75 100 

1 I T l i 1 -h l-i o 



10 



15 



63 66 69 72 
Floral Index 
Ti 



6 9 12 15 
Faunal Index 
Ts 



20 



J*25 



-43 -38 -33 -28 25 50 75 100 



80 ,8 (%o) 



(% rise since 18,000 YBP ) 

SEA LEVEL 



FIGURE A.12 Climatic records of the last 25,000 years, (a) A floral index reflecting 
changes in vegetation in Minnesota, as documented by pollen counts in a bog core 
(Webb and Bryson, 1972). The index is an estimate of July air temperature in °F. (b) A 
faunal index reflecting changes in foraminiferal plankton in a core west of Ireland, from 
C. Sancetta, Brown University (unpublished). The index is an estimate of August sea- 
surface temperature in C. (c) Values of 5 ls O in the ice-core of Camp Century, Greenland 
(Dansgaard et al., 1971). The isotope ratio is judged to reflect air-temperature variations 
over the ice cap, with the more negative values associated with colder temperatures, 
(d) Generalized curve of numerous sea-level records (Bloom, 1971). Chronology of curves 
(a) and (b) is established by "C dates (solid circles) and stratigraphic correlation with 
14 C dates (open circles). Chronology for curve (c) above arrow (12,700 years ago) taken 
from Dansgaard et al., (1971); below arrow, the chronology of Dansgaard et al. has been 
modified by stratigraphic correlation with dated records in North Atlantic deep-sea cores 
(Sancetta et al., 1973). Curve (d) is controlled by numerous "C dates. 



ceding warm interval were destroyed in many places. Such vegetation 
records suggest that by the end of the Younger Dryas event, European 
climate had returned to about its present state. 

The rise in sea level during the last 18,000 years indicated in 
Figure A. 12(d) is generally ascribed to the melting of northern hemi- 
sphere continental ice sheets. Details of the sea-level curve, however, 
do not correspond to the chronology of deglaciation just described: 
while the continental ice sheets had essentially disappeared by about 
7000 years ago, the worldwide stand of sea level has reached its maxi- 
mum only during the last few thousand years or is still slowly rising 



APPENDIX A 



157 



(Bloom, 1971). One possibility is that the West Antarctic ice sheet is 
unstable and has been disintegrating during the entire interval in ques- 
tion. Further research is clearly needed to settle this question, although 
it serves to illustrate the global interrelationships among the elements 
of the climatic system. 

The Last 150,000 Years 

In order to find an ancient counterpart to the warm, ice-free condi- 
tions of the past 10,000 years (the Holocene or present interglacial), 
it is necessary to go back some 125,000 years to an interval known as 
the Eemian interglacial (see Figure A.2). As shown by the proxy data 
of Figure A. 13, the warmest part of this period lasted about 10,000 



NORTH ATLANTIC MEDITERRANEAN 
PLANKTON (Core V2M2) POLLEN 






5 8 


II 


14 


1 ■ J 


15 








30 








45- 








60- 








75 








90" 








105 








120 








135 


^ 


j 


8 


II 


14 



GREENLAND 
ICE CAP 
) (Camp Century Core) (Core P6304-9) 

O BO SO 90 -44 -38 -32 -28 0.5 -02 -09 -L6 -100-70-40-10 



CARIBBEAN SHORELINE FEATURES 

P*y < If N «i S0 J 0PE (Bermuda.Barbadoe.New Guinea) 




FAUNAL INDEX 
T e 



30 60 90 
% ARBOREAL 
POLLEN 



-32 -28 0.5 -0.2 -0.9 -1.6 
ICE CORE %.S0 ,8 in 

SO 18 (%•) PLANKTON SHELL 



100-70 -40 -K> 

SEA LEVEL 

METERS 



FIGURE A.13 Climatic records of the last 135,000 years, (a) A faunal index reflecting 
changes in foraminiferal plankton in a core west of Ireland. The index is an estimate of 
August sea-surface temperature in C (Sancetta et al., 1973). (b) The percentage of 
tree pollen accumulated in a Macedonian lake. High values indicate warmer and some- 
what dryer conditions (van der Hammen et al., 1971). (c) Oxygen isotope ratio expressed 
as 5 ls O in an ice core at Camp Century, Greenland. This is interpreted as indicating 
changing air temperatures over the ice cap (Dansgaard et al., 1971). (d) Oxygen isotope 
ratio in skeletons of planktonic foraminifera in a Caribbean core, interpreted as changes 
in global ice volume. High negative values reflect the melting of ice containing isotopically 
light oxygen (Emiliani, 1968). (e) Generalized sea-level curve. Portion younger than 
20,000 years is representative of a large number of sites (Bloom, 1971); see also Figure 
A.12. Older segments are from elevated coral reef tracts on Barbados, Bermuda, and 
New Guinea (Mesolella et al., 1969; Veeh and Chappell, 1970). Chronology of curves 
(a) to (d) controlled by 14 C dates (solid circles) and by stratigraphic correlation with dated 
horizons (open circles). Curve (e) is controlled by 14 C dates for the portion younger than 
20,000 years and by uranium growth methods for the older segments. 



158 UNDERSTANDING CLIMATIC CHANGE 

years and was followed abruptly by a cold interval of substantial glacial 
growth lasting several thousand years. The interval between this post- 
Eemian event (c. 115,000 years ago) and the most recent glacial maxi- 
mum 18,000 years ago was characterized by marked fluctuations 
superimposed on a generally declining temperature. An intense glacial 
event about 75,000 years ago is sometimes used to separate the interval 
into an older and generally nonglacial regime and a more recent glacial 
one. 

A remarkable feature of the climatic record of the past 150,000 
years is that both the present and the Eemian interglacials began with 
an abrupt termination of an intensely cold, fully glacial interval. Be- 
cause these catastrophic episodes of deglaciation have left such a 
strong imprint on the climatic record, they have been named (in order 
of increasing age) termination I and termination II (see Figure A. 14 
and Broecker and van Donk, 1970) . 

The Last 1,000,000 Years 

For at least the last 1,000,000 years the earth's climate has been char- 
acterized by an alternation of glacial and interglacial episodes, marked 
in the northern hemisphere by the waxing and waning of continental 
ice sheets and in both hemispheres by periods of rising and falling 
temperatures. How clearly these fluctuations are stamped on the various 
proxy data records is shown in Figure A. 14. The most prominent 
features of the isotope curve shown here are seven terminations, mark- 
ing a transition from full glacial to full interglacial conditions. All but 
one (termination III) of these changes are relatively rapid monotonic 
swings and provide an objective basis for defining a climatic "cycle" 
for at least the last 700,000 years. As shown in Figure A. 14, these 
same fluctuations can be recognized in diverse and widely distributed 
records, including the chemical composition of Pacific sediments, fossil 
plankton in the Caribbean, and the soil types in central Europe. These 
"cycles," identified as A to E by Kukla (1970), are found in each of 
the records shown in Figure A. 14 and may be grouped into a climatic 
"regime" covering the last 450,000 years (designated a). The earlier 
records (regime (3) show higher-frequency fluctuations with less co- 
herence among the various proxy climatic recorders. 

The Last 100,000,000 Years 

Although continuous and detailed records are lacking for these earlier 
times, at least the broad outline of this period of climatic history may 



APPENDIX A 



159 



ISOTOPIC COMPOSITION OF 
PACIFIC PLANKTON 
(CORE V28-238) 



CHEMICAL COMPOSITION 
OF EQUATORIAL PACFIC 



TAXONOMIC COMPOSITION 
OF CARIBBEAN PLANKTON 
(CORE VI2-I22) 



CENTRAL EUROPEAN 
SOIL RECORD 




Observation 



-1.5 -2.0 -2.5 100 80 60 40 36.2 36.0 35.8 35.6 

»°' 8 <° / ~ l CoCO, (%) FAUNAL INDCX (S) SOIL TYPE 

Interpretation. DECREASING GLOBAL INCREASING CoCOs DECREASING SALINITY TEMPERATE CLIMATE 



ICE VOLUME 



DISSOLUTION 



FIGURE A.14 Climatic records of the last 1,000,000 years, (a) Oxygen-isotope curve in Pa- 
cific deep-sea core V28-238, interpreted as reflecting global ice volume (Shackleton and 
Opdyke, 1973). The relatively rapid and high-amplitude fluctuations are taken to indicate 
sudden deglaciations and are designated as the terminations I to VII. (b) Calcium 
carbonate percentage in equatorial Pacific core RC1 1-209 (Hays et al., 1969). Low values 
are taken to indicate periods of rapid dissolution by bottom waters, (c) Faunal index 
reflecting changing composition of Caribbean foraminiferal plankton, calibrated as an 
estimate of sea-surface salinity in parts per thousand (Imbrie et al., 1973). Glacial periods 
are marked by the influx of plankton preferring higher-salinity waters (Prell, 1974). (d) 
Sequence of soil types accumulating at Brno, Czechoslovakia (Kukla, 1970). Type 1 is a 
wind-blown loess with a fossil fauna of cold-resistant snails or gley soils indicating 
extremely cold conditions; type 2 includes pellet sands and other hillwash deposits, 
partly interbedded with loess; type 3 includes brownearth and tschernosem soils; type 4 
includes para brownearth (lessive) soils; type 5 are soils of temperate savannas, including 
brownlehms, rubified brownlehms, and rubified lessives with large, hollow carbonate con- 
cretions. The duration of each soil type at this locality is plotted in proportion to the 
maximum thickness observed. Note that each record shown here reflects a climatic 
fluctuation or "cycle" averaging about 100,000 years. This is particularly true during the 
last 450,000 years (climatic regime a). Chronology of the curves is obtained by linear 
interpolation between indicated control points, shown by solid circles. 



be discerned. From the climatic point of view, perhaps the most strik- 
ing aspect of world geography at the beginning of this interval was the 
essentially meridional configuration of the continents and shallow 
ocean ridges, which must have prevented a circumpolar ocean current 
in either hemisphere. In the south this barrier was formed by South 
America and Antarctica (which lay in approximately their present 



160 UNDERSTANDING CLIMATIC CHANGE 

latitudinal positions); by Australia (then a north-eastward extension 
of Antarctica); and by the narrow and relatively shallow ancestral 
Indian Ocean (Dietz and Holden, 1970). About 50,000,000 years ago 
the Antarctica-Australian passage began to open (Kennett et ah, 1973), 
and as Australia moved northeastward, the Indian Ocean widened 
and deepened. Both paleontological and sedimentary evidence suggest 
that about 30,000,000 years ago the Antarctic circumpolar current 
system was first established. This must be considered a pivotal event in 
the climatic history of the past 100,000,000 years, and when the evi- 
dence of global plate movements is complete, it may well be possible 
to account for much of the secular climatic changes of this period as 
a response to the changing boundary conditions imposed by the distribu- 
tion of land and ocean. 

During the last part of the Mesozoic era (from 100,000,000 to 
65,000,000 years ago) global climate was in general substantially 
warmer than it is today, and the polar regions were without ice caps. 
About 55,000,000 years ago numerous geologic records (Addicott, 
1970; Flint, 1971) make it clear that global climate began a long 
cooling trend known as the Cenozoic climatic decline (see Figure A.15). 
Evidence from the marine record indicates that about 35,000,000 years 
ago Antarctic waters underwent a substantial cooling (Douglas and 
Savin, 1973; Shackleton and Kennett, 1974a, 1974b). There is direct 
evidence that ice reached the edge of the continent in the Ross Sea area 
some 25,000,000 years ago; and during the Oligocene epoch, roughly 
35,000,000 to 25,000,000 years ago, global climate was generally 
quite cool (Moore, 1972). During early Miocene time (20,000,000 to 
15,000,000 years ago) evidence from low and middle latitudes indicates 
a warmer climate, but isotopic evidence and faunal data indicate that 
this warming did not affect high southern latitudes. 

About 10,000,000 years ago there is widespread evidence of further 
cooling, substantial growth of Antarctic ice (Shackleton and Kennett, 
1974a, 1974b), and growth of mountain glaciers in the northern 
hemisphere (Denton et al, 1971). For general descriptive purposes the 
present glacial age may be defined as beginning at this time. Indirect 
evidence from marine sediments indicates that about 5,000,000 years 
ago the already substantial ice sheets on Antarctica underwent rapid 
growth and quickly attained essentially their present volume (Shackle- 
ton and Kennett, 1974a, 1974b). This evidence is generally consistent 
with direct records from the Antarctic continent, which show that 
between 7,000,000 and 10,000,000 years ago a large ice sheet existed 
in West Antarctica, and that by about 4,000,000 years ago the ice sheet 
in East Antarctica had developed to essentially its present volume 



APPENDIX A 



161 





m m 


CYCLES, 
CONDITI 

COOLIN 

L(EEMIA 
AXIMUM 


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162 



UNDERSTANDING CLIMATIC CHANGE 



(Denton et al., 1971; Mayewski, 1973). The present Antarctica ice 
mass is equivalent to about 59 m of sea level. 

Although the behavior of the smaller, West Antarctic ice sheet is 
incompletely known, the available evidence indicates that it has under- 
gone considerable fluctuation, and that its variations are roughly syn- 
chronous with the northern hemisphere glacial-interglacial cycle. This 
may be due to the fact that while the East Antarctic ice sheet is solidly 
grounded on the continent, much of the West Antarctic ice mass is 
grounded on islands or on the sea floor and could therefore be sig- 
nificantly influenced by sea-level variations due to glacial changes in 
the northern hemisphere. Such continental ice sheets first appeared 
in the northern hemisphere about 3,000,000 years ago, occupying lands 
adjacent to the North Atlantic Ocean (Berggren, 1972b), and during at 
least the last million years the ice cover on the Arctic Ocean was never 
less than it is today (Hunkins et al, 1971 ) . 

The Last 1,000,000,000 Years 

Our knowledge of the climatic events over this time range consists 
principally of evidence of glaciations as preserved in the geological 
record. This may be seen in perspective with that for the more recent 
periods discussed above in Figure A. 15. The present glacial age is seen 
to be at least the third time that the planet has suffered widespread 
continental glaciation. The Permo-Carboniferous glacial age (about 
300,000,000 years ago) occurred at a time when the earth's land masses 
were joined in a single supercontinent (Pangaea). The area of this 
continent was distributed in roughly equal proportions between the 
hemispheres, with a concentration of land in the midlatitudes (Dietz 
and Holden, 1970). Glaciated portions of Pangaea included parts of 
what are now South America, Africa, India, Australia, and Antarctica. 
One or more earlier glacial ages are known from late Precambrian 
times (about 600,000,000 years ago), from the indications of glacia- 
tion in deposits now widely scattered over the globe, including Green- 
land, Scandinavia, central Africa, Australia, and eastern Asia (Holmes, 
1965). 

Although other glacial ages may have occurred besides those recog- 
nized in Figure A. 15, none has left such a clear and widespread imprint 
on the geological record (Steiner and Grillmair, 1973). While the evi- 
dence is far from complete, it may be that each of the earth's major 
glacial ages — including the present one — resulted from crustal move- 
ments that permitted the development of sharp thermal gradients over 
a continental land mass that includes a pole of rotation. To establish 



APPENDIX A 163 

this or other hypotheses of long-period climatic changes, however, 
will require the assembly of a much more complete geological record 
and the performance of appropriate climate modeling experiments. 



GEOGRAPHIC PATTERNS OF CLIMATIC CHANGE 

While the chronology of certain features of climatic change may be 
revealed by the analysis of instrumental and paleoclimatic data at in- 
dividual sites, the geographic pattern of these changes is an equally 
important characteristic. From what we know of the behavior of the 
(present) atmosphere, it would be remarkable if there were not a defi- 
nite spatial structure to the variations on all climatic times scales. The 
search for these patterns requires synoptic data for the various climatic 
elements, and this is presently available only from the records of 
modern observations and from a few marine proxy sources. 

Structure Revealed by Observational Data 

The task of describing the spatial and temporal structure of climatic 
variations from the observations of the instrumental era is far from 
complete. Most studies have therefore focused primarily on local or 
regional climatic changes. Lamb and Johnson (1961, 1966) have made 
comprehensive analyses of intrahemispheric and interhemispheric cli- 
matic indices, and the statistical structure of these circulation variations 
has been studied by Willett (1967), Wagner (1971), Iudin (1967), 
Brier (1968), and Kutzbach (1970). Such analyses, especially of 
hemispheric pressure data, reveal that the year-to-year and decade-to- 
decade variations have a spatial structure that may be associated with 
amplitude and phase changes of the long planetary waves in the 
atmosphere. 

The essentially two-dimensional character of climate is masked in 
studies of zonally averaged parameters, although these may be use- 
ful for other purposes. An example of the importance of both zonal 
and nonzonal spatial variability of the atmospheric circulation is 
provided by the first eigenvector pattern (or empirical orthogonal 
function) of hemispheric pressure for January, shown in Figure A. 16, 
as well as by the patterns of pressure, temperature, and rainfall variabil- 
ity shown in Figures A. 17, A. 18, and A. 19. These data suggest an 
association between the changes in the monthly average intensity and 
position of the Aleutian and Icelandic lows. For example, during the 
first two decades of this century there has been a tendency for decreased 
intensity and westward extension of the Aleutian low, coupled with an 



164 



UNDERSTANDING CLIMATIC CHANGE 




JANUARY 
RRST EIGENVECTOR 



FIGURE A.16 The first eigenvector of northern hemisphere sea-level pressure, based on 
the individual mean January maps for 1899-1969 (Kutzbach, 1970). This spatial function 
accounts for 22 percent of the total inter-January variance. 



increased intensity and northeastward shift of the Icelandic low. Lamb 
(1966) and Namias (1970) have described important regional changes 
in temperature and precipitation associated with these circulation 
changes. The opposite tendency has prevailed since the mid-1 950's, 
and Lamb (1966), Winstanley (1973), and Bryson (1974) have 
described the possible relationships between the changing midlatitude 
circulation patterns of the 1960's, the equatorial shift of the subtropical 
highs, and the increasing frequency of droughts along the southern 
fringes of the monsoon lands of the northern hemisphere (see Figure 
A.8). These changes appear to reflect an equatorward extension of 
the westerly wave regime and a contraction of the Hadley circulation, 



APPENDIX A 



165 




FIGURE A.17 Standard deviation (mbar) of monthly mean pressure at sea level, 1951- 
1966 (Lamb, 1972). (a) December, northern hemisphere; (b) July, southern hemisphere. 



166 



UNDERSTANDING CLIMATIC CHANGE 




T 
December 



FIGURE A.18 Standard deviation (°C) of monthly mean surface air temperature in the 
northern hemisphere, 1900-1950 (Lamb, 1972). (a) July; (b) December. 



APPENDIX A 



167 




Rainfall variability 

The figures denote percentage departures from normal 

Under 10 10-15 15-20 20-25 25-30 30-40 Over 40% 



FIGURE A.19 The variability of mean annual rainfall for the world (adapted from 
Petterssen, 1969). 



although much further analysis is clearly required to confirm such a 
conjecture. 

Interhemispheric relationships of climatic indices have been (and 
remain) less amenable to study because of the general lack of observa- 
tions from the southern hemisphere. Observations are sufficient, how- 
ever, to show that the circulation in the southern hemisphere is some- 
what stronger and steadier than that in the northern hemisphere. Whether 
this results in the southern hemisphere circulation leading that in the 
northern hemisphere, or whether variable features in the equatorial 
circulation influence both hemispheres similarly, is not presently known 
(Bjerknes, 1969b; Fletcher, 1969; Lamb, 1969;Namias, 1972a, 1972b). 
It is likely that interhemispheric relationships of one sort or another 
are important for the understanding of climatic variations, and that 
our ability to describe them will require the availability of much more 
comprehensive data than now exist from the southern hemisphere, the 
equatorial region, and the oceanic and polar regions of the northern 
hemisphere. 

The present accumulation of upper-air data, especially in the north- 
ern hemisphere since the early 1950's, however, has permitted a 
beginning of the study of the three-dimensional spatial and temporal 
variability of the general circulation. A foundation of basic statistics is 



168 



UNDERSTANDING CLIMATIC CHANGE 



provided by calculations of the means and variances of standard 
meteorological variables (see, for example, Crutcher and Meserve, 1970; 
Taljaard et al, 1969) and by atlases of energy budgets (Budyko, 1963). 
The covariance structure of circulation patterns at 700 mbar in the 
northern hemisphere is treated by O'Connor (1969), and other aspects 
of the tropospheric circulation have been considered by Gommel (1963) 
and Wahl (1972). The most comprehensive analysis of atmospheric 
circulation statistics, however, is that based on the period 1958-1963 
as undertaken by Oort and Rasmusson (1971). While this work docu- 
ments the monthly, seasonal, and annual variations of many features 
of the observed general circulation (in the northern hemisphere), it 
does not directly address many of the variables of primary climatic 
interest. Using the same data set, however, Starr and Oort (1973) 
have reported an unmistakable downward trend of the mean air 
temperature in the northern hemisphere of 0.6 °C over the five-year 
interval shown in Figure A. 20. Diagnostic studies of this type represent 
great investments of time and effort but are essential steps toward the 
monitoring of climate and an assessment of the mechanisms of climatic 
variation. 

A complete description of climatic changes from instrumental records 
must also include studies of the momentum and energy budgets of the 
atmosphere and oceans and their variability with time over many years 
and decades. While this must remain largely a task for the future, 
several efforts have established the existence of significant interannual 
variations in the atmosphere. Krueger et al. (1965) have discussed the 



May 
1958 



April 
1959 



April 
1960 



April 
1962 



April 
1963 




1959 



1960 



1961 1962 



1963 



FIGURE A.20 Monthly mean mass-averaged values of the northern hemisphere tempera- 
ture for the period May 1958 to April 1963 (Starr and Oort, 1973). The consecutive 
monthly averages are plotted on the scale marked at the top; the bottom scale shows 
the beginning of each calendar year. 



APPENDIX A 169 

interannual variations of available potential energy, and Kung and Soong 
(1969) have described the fluctuations of the atmospheric kinetic 
energy budget. As noted previously, the interannual variations of pole- 
ward angular momentum and energy fluxes has been studied compre- 
hensively by Oort and Rasmusson ( 1971 ). A measure of this variability 
is shown in Figure A. 21 and amounts to about ±30 percent of the 
mean transports. 



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5-year mean for 1959-1963. (a) Angular momentum transport; (b) sensible heat transport. 



170 



UNDERSTANDING CLIMATIC CHANGE 



The unique global potential of satellite-based measurements has 
been exploited by Vonder Haar and Suomi (1971), who have sum- 
marized the satellite measurements of planetary albedo and of the 
planetary radiation budget for the five years 1962 to 1966. They found 
large interannual variations in the zonally averaged equator-to-pole 
gradient of the net radiation as shown in Figure A. 22. This forcing 
function can now be monitored routinely by meteorological satellites 
and opens the door to more detailed studies of atmospheric energetics 
than heretofore possible (Winston, 1969). Vonder Haar and Oort 
(1973) have combined satellite measurements of the earth's radiation 
budget with atmospheric energy transport calculations to produce a new 
estimate of the poleward energy transport by the northern hemisphere 



0.3 


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Q 
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fc °' 2 

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variation (vertical bars) of the equator-to-pole gradient of 
net radiation as measured from satellites (Vonder Haar 
and Suomi, 1971). (a) Northern hemisphere; (b) southern 
hemisphere. 



APPENDIX A 



171 



oceans. They find that the oceanic heat transport averages about 40 per- 
cent of the total in the 0-70 °N latitude band and accounts for more 
than half at many latitudes. Another example of the use of satellite- 
derived measurements of climatic indices is given by Kukla and Kukla 
(1974). Their measurements of the interannual changes in the area 
of snow and ice cover in the northern hemisphere are shown in Figure 
A. 23 and reveal year-to-year fluctuations of the order of a few percent. 
Note, however, the relatively large change during 1971 and the subse- 
quent maintenance of extensive snow and ice coverage and an associated 
increase of the reflected solar radiation. 

Time variations of the surface energy budget on a global scale are 




1969 



1970 



1971 



1972 



1973 



FIGURE A.23 Twelve-month running means of snow and ice cover in the 
northern hemisphere (upper curve) and the estimated reflected solar 
radiation disregarding variations of cloudiness (lower curve), as reported 
by Kukla and Kukla (1974). The averages are plotted on terminal dates, with 
the years marking January 1. 



172 UNDERSTANDING CLIMATIC CHANGE 

not available from direct observations and must be inferred from the 
conventional measurements of temperature, humidity, cloudiness, wind, 
and radiation. Fletcher (1969) has drawn attention to the variations in 
the energy budget of polar regions as a function of variable sea-ice con- 
ditions, while Sawyer (1964) has noted the possible role of fluctuations 
in the surface energy budget as a cause of interannual variations of the 
general circulation itself. 

A number of observational studies of large-scale interaction between 
the ocean and the atmosphere have illustrated the complexity and im- 
portance of this mechanism; see, for example, Weyl (1968) and Lamb 
and Ratcliffe (1972). Bjerknes (1969b) has considered the response 
of the North Pacific westerlies to anomalies of equatorial sea-surface 
temperature and variations in the Hadley circulation, while Namias 
(1969, 1972b) has described positive feedback relationships between 
large-scale patterns of ocean-surface temperature in midlatitudes and 
the circulation of the overlying atmosphere. Such modes of atmosphere- 
ocean coupling may be important parts of climatic fluctuations and must 
be given further study. 

In summary, we may say that observational data at the earth's sur- 
face show that during the period 1900 to 1940 the northern hemisphere 
as a whole warmed, although some areas (mainly the Atlantic sector of 
the Arctic and northern Siberia) warmed far more than the global 
average, some areas became colder, and others showed little measurable 
change (Mitchell, 1963). In the time since 1940, an overall cooling 
has occurred but is again characterized by a geographical structure; 
cooling since 1958 has occurred in the subtropical arid regions and in 
the Arctic (Starr and Oort, 1973). There is also some evidence that 
the northern hemisphere oceans are cooling (Namias, 1972b), although 
the oceanic data base necessary to confirm this has not yet been 
assembled. 



Structure Revealed by Paleoclimatography 

Most of the work done to date on climatic change beyond the time 
frame encompassed by meteorological observations represents a study 
of time series taken at specific sites. This lack of synoptic data on the 
longer-range climatic changes is a serious handicap to the portrayal 
and understanding of the mechanisms involved. In order to underscore 
these points, and to encourage further research, we present here ex- 
amples of the few proxy data that have been assembled to reveal a 
spatial structure of climatic change. 



APPENDIX A 



173 



Distribution of Ice Sheets 

The continental margins of the northern hemisphere ice sheets at their 
maximum extension during the last million years are clearly marked 
by the debris deposits in terminal moraines, while the extent of sea 
ice is recorded by features preserved in marine sediments. Figure A. 24 



140 



160 



180 



160 



140 




FIGURE A.24 Maximum extent of northern hemisphere ice cover during the present 
glacial age (modified after Flint, 1971). Continental ice sheets, indicated by the dotted 
area, are B, Barents Sea; S, Scandinavian; G, Greenland; L, Laurentide; C, Cordilleran. 
Sea ice is indicated by the cross-hatched pattern. The boundary mapped is the southern- 
most extent of the ice margin that occurred in any sector during the last million years. 
The last glacial maximum, about 18,000 years ago, occupied about 90 percent of the 
area shown here (see also Table A.2). 



174 UNDERSTANDING CLIMATIC CHANGE 

shows the distribution of maximum ice cover, and Table A. 2 gives 
statistics of the areas of the individual continental ice sheets. In North 
America the ice extended as far south as 40 °N and spanned the entire 
width of the continent, while in Europe the ice sheet extended only to 
about 50 °N. Note that large regions in eastern Siberia were unglaciated. 



Sea-Surface Temperature Patterns 

The north-south migration of polar waters in the North Atlantic in 
response to major cycles of glaciation is shown in Figure A.25. During 
glacial maxima these waters were found as far south as 40 °N, well 
beyond the present extent of polar waters. A synoptic analysis of the 
ocean surface temperatures of 18,000 years ago (at about the time of 
the last glacial maximum) is shown in Figure A. 26. These temperature 
estimates have been derived by multivariate statistical techniques applied 
to planktonic organisms as preserved in about 100 deep-sea cores in 
scattered locations across the North Atlantic (Mclntyre et al., 1974). 
The most striking feature of this glacial-age map is the extensive south- 
ward displacement of the 10 to 14°C water, while the warmer water 
was found in nearly its present position. In parts of the Sargasso Sea 
the glacial-age ocean was, if anything, slightly warmer than it is today. 

Because the atmosphere receives much of its heat from the sea, such 
estimates of sea-surface temperature are likely to be important in de- 
veloping a satisfactory reconstruction of past climates, and it is there- 
fore important to consider their reliability. Berger (1971), for ex- 
ample, has suggested that carbonate dissolution on the sea bed may 
distort the taxonomic composition of the fossil fauna on which such 
paleotemperature estimates are based. Kipp (1974), on the other hand, 
shows that when the statistical transfer functions are calibrated on ma- 
terials that incorporate the dissolution effects, an unbiased estimate of 
such parameters as the sea-surface temperature can be obtained. The 
temperature reconstruction in Figure A.26(b) is based on the statistics 
of the foraminiferal fauna distribution and encompasses 91 percent of 
the variance of the data (Mclntyre, 1974). The 80 percent confidence 
interval of each of the cores is ±1.8°C (Kipp, 1974). Shackleton and 
Opdyke (1973), using a revised isotopic method based on the differ- 
ence between ls O values in benthic and planktonic species, have pro- 
vided an independent confirmation of the sea-surface temperature 
estimates of Imbrie et al. (1973) for a portion of the glacial-age 
Caribbean. 

Other reconstructions of paleo-ocean surface temperatures have been 
based on data from radiolaria, coccoliths, and foraminifera; and al- 




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176 



APPENDIX A 177 

though some discrepancies are revealed where independent data are 
available, the derived ocean temperatures show considerable spatial 
coherence (Mclntyre, 1974). Such estimates of past sea-surface tem- 
perature will prove useful in climatic simulations with numerical general 
circulation models (see Appendix B). 



Patterns of Vegetation Change 

Figure A.27 illustrates the use of fossil pollen data to record the changes 
in vegetation accompanying the deglaciation of eastern North America 
during the interval 11,000 to 9000 years ago. At the beginning of 
this time, pine species occupied sites in the southeastern Appalachians, 
but as the ice retreated, the pine moved farther north and west to 
colonize newly uncovered areas. A relatively complete chronology of 
the retreat of the Laurentide ice sheet itself is given by radiocarbon 
dating (Brysonef al, 1969). 

Patterns of Aridity 

For only four desert areas in the world do we have enough information 
to plot aridity as a function of time, and even in these areas the record 
extends back only a few tens of thousands of years. As shown in 
Figure A.28, the data suggest a degree of synchroneity between the two 
African regions and the Great Basin, while the records from the Middle 
East are quite different. None of the data from closed-basin lakes show 
significant correlation with the glacial record, and we are clearly a long 
way from understanding the response of arid regions to glacial cycles. 
More generally, insufficient research has been devoted to the role of 
desert regions in the processes responsible for the climate of the earth. 

Patterns of Tree-Ring Growth 

Changes of thickness of the growth rings added by trees each year 
reflect environmental change in a complex way. By appropriate calibra- 
tion, such data may be made to furnish significant climatic information 
for the past several hundred to several thousand years. Studies of many 
tree-ring series over a wide geographic area can, moreover, provide 
accurately dated synoptic evidence of regional climatic patterns (Fritts, 
1965). 

Fritts et al. (1971) have demonstrated the feasibility of reconstruct- 
ing the anomalies of sea-level pressure and temperature from the 
spatial patterns of tree growth over western North America. Examples 



178 



UNDERSTANDING CLIMATIC CHANGE 

95 85 75 65 




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FIGURE A.27 The distribution of pine pollen at selected times dur- 
ing the deglaciation of eastern North America (Bernabo et al., 1974). 
Contours are lines of pollen frequency, expressed as a percent of 
total pollen. Control points representing radiocarbon dated cores 
are indicated by the open circles. The approximate margins of the 
Laurentide ice sheet are indicated by the stippled pattern (after 
Bryson et al., 1969). 



APPENDIX A 



179 



LARGE 



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50 



FIGURE A.28 Variations in the size of closed-basin lakes, as indicated by the degree 
of aridity found from radiocarbon dating of shorelines and bottom sediments. Higher 
rainfall and lower evaporation may be inferred at the times of larger water surfaces, 
(a) From Broecker and Kaufman (1965); (b) from Kaufman (1971); (c) from Butzer et al. 
(1972); (d) from Servant et al. (1969). 



of such synoptic maps based on average decadal growth are given in 
Figure A. 29. Although such reconstructions show considerable varia- 
tion in the year-to-year climatic states, the inferred variations in the 
intensity of Icelandic and Aleutian lows, for example, are similar to 
those described in the modern record (Kutzbach, 1970). The develop- 
ment of an expanded network of tree-ring sites could significantly 
broaden our knowledge of the patterns of climatic fluctuations over the 
past several centuries. 



SUMMARY OF THE CLIMATIC RECORD 

In this survey of past climates, the characteristic time and spatial 
structures of climatic variations have been discussed as though there 
were sufficient data to document large regions of the globe. This is true 
only for the more recent parts of the instrumental period, as there are 
large gaps in the presently available historical and proxy climatic 
records. With these limitations in mind, it is nevertheless useful to 
summarize the general characteristics of the climatic record: 



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180 



APPENDIX A 181 

1. The last postglacial thermal maximum was reached about 6000 
years ago, and climates since then have undergone a gradual cooling. 
This trend has been interrupted by three shorter periods of more marked 
cooling, simliar to the so-called Little Ice Age of a.d. 1430-1850, each 
followed by a temperature recovery. The well-documented warming 
trend of global climate beginning in the 1880's and continuing until the 
1940's is a continuation of the warming trend that terminated the 
Little Ice Age. Since the 1940's, mean temperatures have declined and 
are now nearly halfway back to the 1 880 levels. 

2. Climatic changes during the past 20,000 years are as severe as 
any that occurred during the past million years. At the last glacial maxi- 
mum, extensive areas of the northern hemisphere were covered with 
continental ice sheets, sea level dropped about 85 m, and sea-surface 
temperatures in the North Atlantic fell by as much as 10°C. At 
northern midlatitude sites not far from the glacial margins (locations 
now occupied by major cities and extensive agricultural activity), air 
temperatures fell markedly, drastic changes occurred in the precipita- 
tion patterns, and wholesale migrations of animal and plant communi- 
ties took place. 

3. The present interglacial interval — which has now lasted for about 
10,000 years — represents a climatic regime that is relatively rare during 
the past million years, most of which has been occupied by colder, 
glacial regimes. Only during about 8 percent of the past 700,000 years 
has the earth experienced climates as warm as or warmer than the 
present. 

4. The penultimate interglacial age began about 125,000 years ago 
and lasted for approximately 10,000 years. Similar interglacial ages — 
each lasting 10,000 ±2000 years and each followed by a glacial maxi- 
mum — have occurred on the average every 100,000 years during at 
least the past half million years. During this period fluctuations of the 
northern hemisphere ice sheets caused sea-level variations of the order 
of 100 m. In contrast, the East Antarctic ice sheet has apparently varied 
little since reaching its present size about 5 million years ago, while 
the West Antarctic ice sheet appears to have been disintegrating for 
many thousands of years. 

5. About 65 million years ago global climates were substantially 
warmer than today, and subsequent changes may be viewed as part of 
a very long-period cooling trend. For even earlier times, the proxy 
climatic evidence becomes increasingly fragmentary. The best docu- 
mented records suggest two previous extensive glaciations, occurring 
about 300 million and 600 million years ago. 



182 UNDERSTANDING CLIMATIC CHANGE 

FUTURE CLIMATE: SOME INFERENCES FROM PAST BEHAVIOR 

The overall picture of past climatic changes described in this survey 
suggests the existence of a hierarchy of fluctuations that stand out 
above the "white noise" or random fluctuations presumed to exist on all 
time scales. In addition to the dominant period of about 100,000 years, 
there are apparent quasi-periodic fluctuations with time scales of about 
2500 years and shorter-period fluctuations on the order of 100-400 
years. Each of these explains progressively less of the total variance 
but may nevertheless be climatically significant. No periodic component 
of climatic change on the order of decades has yet been clearly estab- 
lished, although significant excursions of climate are observed to occur 
in anomalous groups of years. 

In view of the limited resolving power of most climatic indicators, 
especially those for the relatively remote geological past, it is difficult 
to establish whether the apparent fluctuations are quasi-periodic or 
whether they more nearly represent what are basically random Markov- 
ian "red-noise" variations. In the case of the longer-period variations 
(of 100,000-year and 20,000-year periods), there is circumstantial evi- 
dence to suggest that these may have been induced in some manner 
by the secular variations of the earth's orbital elements, which are known 
to alter the seasonal and latitudinal distribution of solar radiation re- 
ceived at the top of the atmosphere. In other cases, the observed varia- 
tions have yet to be convincingly related to any external climatic control. 
The mere existence of such variations does not necessarily mean that 
changes in the external boundary conditions are involved, however. The 
internal dynamics of the climatic system itself may well be the origin 
of some of these features. Whether forced or not, climatic behavior of 
this type deserves careful study, as the conclusions reached bear directly 
upon the problem of inferring the future climate. 

The prediction of climate is clearly an enormously complex prob- 
lem. Although we have no useful skill in predicting weather beyond a 
few weeks into the future, we have a compelling need to predict the 
climate for years, decades, and even centuries ahead. Not only do we 
have to take into account the complex year-to-year changes possibly 
induced by the internal dynamics of the climatic system, and the likely 
continuation of the (yet unexplained) quasi-periodic and episodic 
fluctuations of the last few thousand years discussed above, but also 
the changes induced by possibly even less predictable factors such as the 
aerosols added to the atmosphere by volcanic eruptions and by man 
himself (Mitchell, 1973a, 1973b). These questions lie at the heart of 



APPENDIX A 183 

the problem of climatic variation and are given consideration elsewhere 
in this report. 

In the face of these uncertainties, any projection of the future climate 
carries a great risk. Nevertheless, we may speculate about the possible 
course of global climate in the decades and centuries immediately 
ahead by making certain assumptions about the character of the major 
fluctuations noted in the climatic record. In the following paragraphs 
we attempt to draw together these considerations into an overall assess- 
ment of the probable direction and magnitude of present-day climatic 
change, taking into account the risk of a major future change associated 
with the seemingly inevitable onset of the next glacial period. 

Potential Contribution of Sinusoidal Fluctuations of Various Time 
Scales to the Rate of Change of Present-Day Climate 

Estimates of the amplitudes of all the principal climatic fluctuations 
identified in this report are listed in Table A. 3 (where they have been 
made consistent with the data presented in Figure A. 2 and are expressed 
in terms of the total range of temperature between maxima and minima). 
On the assumption that all of these fluctuations can be approximated 
by quasi-periodic sine waves, the ratio of the amplitude (A) to the 
period (P) of each fluctuation becomes proportional to the maximum 
contribution of that fluctuation to the rate of change of climate. By 
considering also the phase of each fluctuation, as inferred from the 
paleoclimatic record, the contribution of each fluctuation to the present- 
day rate of change can be estimated (see Table A.3). 

Estimation of the phase of each sinusoidal fluctuation (indicated by 
the estimated dates of the last temperature maximum in Table A.3) 
permits an assessment of the sign and magnitude of the contribution of 
each fluctuation to the total rate of change of globally average tempera- 
ture in the present epoch. The sum of these individual contributions 
( -0.015°C/yr) agrees reasonably well with the observed rate of change 
of — 0.01°C/yr during the past two decades, as determined from 
analyses of surface climatological data by Reitan ( 1971 ) and by Budyko 
(1969). It should be noted that this trend is dominated by the shortest 
fluctuations, and especially by the fluctuations of the order of 100 years 
(see Figure A.6). 

The estimated maximum rate of change associated with all time 
scales of climatic fluctuation shown in Figure A. 2 is plotted as a con- 
tinuous function of wavelength in Figure A. 30. The family of curves 
also shown in this figure indicates the relationship between maximum 



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184 



APPENDIX A 



185 




.001 - 



.0001 



.00001 

100,000 



10,000 1,000 100 10 

PERIOD OF FLUCTUATION (years) 



FIGURE A.30 Relative (maximum) rate of change of climate contributed by climatic 
fluctuations, as a function of characteristic wavelength. The family of parallel curves 
shows the expected relationship in Markovian "red noise" as characterized by the serial 
correlation coefficient at a lag of one year. The dashed line is a conservative estimate 
of actual climate as inferred from the data of Figure A.2 and from additional data on the 
shorter-period fluctuations from Kutzbach and Bryson (1974). The dotted curve shows 
the modifications to be expected if the principal fluctuations identified in Table A. 3 were 
actually quasi-periodic. 



rate of change and wavelength in Markovian "red noise," for various 
degrees of "redness" characterized by the value of the serial correla- 
tion coefficient at a time lag of one year (Gilman et al., 1963). By com- 
parison with these curves, it is suggested that the observed shorter- 
period climatic fluctuations (i.e., fluctuations of the order of 100 to 
200 years) are not clearly distinguishable from random fluctuations, 
whereas the longer-period fluctuations (especially those with periods 
of 20,000 years or more) may be appreciably larger in amplitude than 
would be expected in random noise. The contributions of the longer- 
period fluctuations to present-day climatic change are seen nonetheless 



186 UNDERSTANDING CLIMATIC CHANGE 

to be relatively small. Should the longer-period fluctuations be non- 
sinusoidal (or episodic) in form, rates of change perhaps ten times 
larger than the magnitudes shown in Figure A. 30 could be possible. 
Even such rates, however, would contribute little over and above the 
normal interannual variability of present-day global climate, and the 
cumulative change of climate associated with the longer-period fluctua- 
tions would remain relatively small until several centuries had elapsed. 

Despite its simplistic view of climatic change, this exercise is an 
instructive one in that it demonstrates how difficult it would be for 
longer-period sinusoidal fluctuations to contribute substantially to the 
changes of climate taking place in the twentieth century. If the longer- 
period fluctuations are those that primarily determine the course of 
the glacial-interglacial succession of global climate, it would seem 
that the transition to the next glacial period — even if it has already 
commenced — will require many centuries to accumulate to a drastic 
shift from present climatic conditions. 

In assessing such projections, however, we must keep in mind that 
our ability to anticipate the locally important synoptic pattern of climatic 
variations is limited. The work of Mitchell (1963), for example, has 
shown that while the northern hemisphere average air temperatures 
rose only about 0.2 °C during the period 1900 to 1940, there were 
many areas that deviated markedly from this hemispheric average 
trend. Parts of the eastern United States, for example, exhibited a 
1.0 °C rise in average temperature (5 times the hemispheric average), 
parts of Scandinavia and Mexico showed temperature increases of 
2.0°C (10 times the hemispheric average), while in Spitsbergen the 
warming was 5°C (25 times the hemispheric average). The correspond- 
ing data on other climatic elements are sparse but may be expected to 
exhibit comparable or even greater spatial variance. 

Likelihood of a Major Deterioration of Global Climate in the 

Years Ahead 

As noted above, the longer-period climatic fluctuations seem to be 
associated with larger amplitudes of change than those consistent with 
Markovian "red-noise" behavior. The same cannot be said, however, of 
the shorter-period fluctuations. For the moment let us suppose that 
all the fluctuations described in this report are actually random fluctua- 
tions, in the sense that transitions between successive maxima and 
minima may occur at random (Poisson-distributed) intervals of time 
rather than at more or less regular intervals. The probability that one 
or more transitions of a fluctuation will occur in an arbitrarily specified 



APPENDIX A 



187 



length of time may then be calculated from the negative binomial distri- 
bution. Following this approach, we can assess the risk of encountering 
a change of climate in the years ahead as rapid as the maximum rate 
of change otherwise associated with sinusoidal climatic fluctuations 
on each of the characteristic time scales noted above. Such a measure 
of risk, for time intervals between 1 year and 1000 years into the future, 
can be inferred by interpolation between the curves of transition prob- 
ability in Figure A.31. The proper interpretation of this figure will 
be apparent from the following examples : 




0.01 



0.001 



10 100 

WAITING TIME(YEARS) 



1000 



FIGURE A.31 Probability of onset of climatic transitions analogous to the changes 
between maxima and minima in climatic fluctuations of arbitrarily selected characteristic 
wavelengths (interior numbers, in years), as a function of elapsed time after present. 
Dashed curves denote probability of one transition; solid curves denote that of one or 
more transitions. Based on the assumption that intervals between transitions are strictly 
random (Poisson distributed). 



188 UNDERSTANDING CLIMATIC CHANGE 

1. The curve labeled 100,000 in the figure indicates the probability 
of a major transition of climate (in either direction) that is normally 
associated with climatic fluctuations on the time scale of 100,000 years 
(a change of global average temperature of up to perhaps 8°C in a 
total time interval of 50,000 years or less). The curve indicates that 
if successive transitions of this kind recur at random time intervals as 
assumed here, the onset (or termination) of such a transition will occur 
in the next 100 years with a probability of about 0.002 and in the next 
1000 years with a probability of about 0.02. 

2. The dashed curve labeled 100 in the figure indicates the prob- 
ability of one transition of climate (in either direction) that is normally 
associated with climatic fluctuations on the time scale of 100 years (a 
change of up to perhaps 0.5 °C in a total time interval of about 50 years 
or less). Such a transition is indicated to have a probability of about 
0.02 of occurring in the next year, a probability of about 0.16 of occur- 
ring in the next 10 years, and a probability of about 0.35 of occurring 
in the next 50 years. The solid line labeled 100 in the figure indicates 
the probability of one or more transitions of the same kind, which 
rises from about 0.2 in the next 10 years to about 0.8 in the next 
100 years. If it can be assumed that the typical duration of such a 
transition (when it occurs) is not less than four or five decades, and 
that only one such transition can occur at the same time, then the dashed 
curve would be the appropriate guide for estimating such probabilities 
in the next few decades. Otherwise, the solid curve would be a more 
appropriate guide. 

When Figures A. 30 and A. 31 are considered together, it is suggested 
that whether climatic fluctuations are or are not quasi-periodic, those 
that are most relevant to the course of global climate in the years and 
decades immediately ahead are the shorter-period (historical) fluctua- 
tions and not the longer-period (glacial) fluctuations. Even if the phase 
of the longer-period changes is such as to contribute to a cooling of 
present-day climate, the contribution of such fluctuations to the rate 
of change of present-day climate would seem to be swamped by the 
much larger contributions of the shorter-period (if more ephemeral) 
historical fluctuations. We must remember, however, that this analysis 
assumes a simple model of climatic change in which climatic fluctua- 
tions of various periods are independent and therefore additive. The 
paleoclimatic record presented here does not preclude the possibility 
that relatively sudden climatic changes could arise through interactions 
between fluctuations of different periods. 

One may still ask the question: When will the present interglacial 
end? Few paleoclimatologists would dispute that the prominent warm 



APPENDIX A 189 

periods (or interglacials) that have followed each of the terminations 
of the major glaciations have had durations of 10,000 ±2000 years. In 
each case, a period of considerably colder climate has followed im- 
mediately after the interglacial interval. Since about 10,000 years has 
elapsed since the onset of the present period of prominent warmth, the 
question naturally arises as to whether we are indeed on the brink of a 
period of colder climate. Kukla and Matthews (1972) have already 
called attention to such a possibility. There seems little doubt that the 
present period of unusual warmth will eventually give way to a time 
of colder climate, but there is no consensus with regard to either the 
magnitude or rapidity of the transition. The onset of this climatic de- 
cline could be several thousand years in the future, although there is 
a finite probability that a serious worldwide cooling could befall the 
earth within the next hundred years. 

What is the nature of the climatic changes accompanying the end 
of a period of interglacial warmth? From studies of sediments and soils, 
Kukla finds that major changes in vegetation occurred at the end of the 
previous interglacial (Figure A. 14). The deciduous forests that covered 
areas during the major glaciations were replaced by sparse shrubs, and 
dust blew freely about. The climate was considerably more continental 
than at present, and the agricultural productivity would have been 
marginal at best. The stratification of fossil pollen deposits in eastern 
Macedonia (Figure A.13) also clearly shows a marked change in vege- 
tative cover between interglacial warmth and the following cold periods. 
The oak-pine forest that existed in the area gave way to a steppe shrub, 
and grass was the dominant plant cover. Other evidence from deep-sea 
cores reveals a substantial change in the surface water temperature in 
the North Atlantic between interglacial and glacial periods (Figure 
A.13), and the marine sediment data show that the magnitude of the 
characteristically abrupt glacial cooling was approximately half the total 
glacial to interglacial change itself. 

The question remains unresolved. If the end of the interglacial is 
episodic in character, we are moving .toward a rather sudden climatic 
change of unknown timing, although as each 100 years passes, we have 
perhaps a 5 percent greater chance of encountering its onset. If, on 
the other hand, these changes are more sinusoidal in character, then 
the climate should decline gradually over a period of thousands of years. 
These are the limits that we can presently place on the nature of this 
transition from the evidence contained in the paleoclimatic record. 

These climatic projections, however, could be replaced with quite 
different future climatic scenarios due to man's inadvertent interference 
with the otherwise natural variation (Mitchell, 1973a). This aspect of 



190 UNDERSTANDING CLIMATIC CHANGE 

climatic change has recently received increased attention, as evidenced 
by the smic report (Wilson, 1971). A leading anthropogenic effect is 
the enrichment of the atmospheric C0 2 content by the combustion of 
fossil fuels, which has been rising about 4 percent per year since 1910. 
There is evidence that the ocean's uptake of much of this C0 2 is 
diminishing (Keeling et al., 1974), which raises the possibility of even 
greater future atmospheric concentrations. Man's activities are also con- 
taminating the atmosphere with aerosols and releasing waste heat into 
the atmosphere, either (or both) of which may have important climatic 
consequences (Mitchell, 1973b). Such effects may combine to offset a 
future natural cooling trend or to enhance a natural warming. This 
situation serves to illustrate the uncertainty introduced into the prob- 
lem of future climatic changes by the interference of man and is occur- 
ring before adequate knowledge of the natural variations themselves 
has been obtained. Again, the clear need is for greatly increased re- 
search on both the nature and causes of climatic variation. 



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Shackleton, N. J., and J. P. Kennett, 1974b: Paleotemperature history of the 

Cenozoic and the initiation of Antarctic glaciation; oxygen and carbon isotope 

analyses in dsdp Sites to 77, 279, and 281, Initial Reports of the Deep Sea 

Drilling Project, 29 (in press). 
Shackleton, N. J., and N. D. Opdyke, 1973: Oxygen isotope and paleomagnetic 



APPENDIX A 195 

stratigraphy of equatorial Pacific core V28-238: oxygen isotope temperatures 
and ice volumes on a 10 5 and 10° year scale, Quaternary Res., 5:39-55. 

Starr, V. P., and A. H. Oort, 1973: Five-year climatic trend for the Northern 
Hemisphere, Nature, 242:310-313. 

Steiner, J., and E. Grillmair, 1973: Possible galactic causes for periodic and epi- 
sodic glaciation, Geol. Soc. Am. Bull., 84: 1003-1018. 

Suess, H. E., 1970: The three causes of secular C" fluctuations, their amplitudes 
and time constants, in Radiocarbon Variations and Absolute Chronology, Nobel 
Symposium 12, 1. U. Olsson, ed., Wiley, New York, pp. 595-604. 

Sverdrup, H. U., M. W. Johnson, and R. H. Fleming, 1942: The Oceans, Prentice- 
Hall, Englewood Cliffs, N.J., 1087 pp. 

Taljaard, J. J., H. Van Loon, H. L. Crutcher, and R. L. Jenne, 1969: Climate of 
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Van der Hammen, T., T. A. Wijmstra, and W. H. Zagwijn, 1971: The floral record 
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Veeh, H. H., and J. Chappell, 1970: Astronomical theory of climatic change: sup- 
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Vonder Haar, T. H., and A. H. Oort, 1973: New estimates of annual poleward 
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Wagner, A. J., 1971: Long-period variations in seasonal sea-level pressure over the 
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PhD dissertation, Dept. of Meteorol. and Oceanog., NYU, New York, 152 pp. 



APPENDIX B 

SURVEY OF THE CLIMATE SIMULATION 
CAPABILITY OF GLOBAL CIRCULATION MODELS 



INTRODUCTION 

Much of the present effort within garp, as well as other research pro- 
grams in the atmospheric and oceanic sciences, is aimed toward the 
development of a quantitative understanding of the behavior of the 
atmosphere, with the immediate objective of improving the accuracy of 
weather forecasts. Other research efforts and plans, and the research 
program proposed in this report in particular, are directed to the 
longer-range objective of understanding the physical basis of climate 
and climatic change. Essential to both of these objectives are the dy- 
namical models of the global atmospheric and oceanic circulation. 

These general circulation models (or gcm's) have been developed 
over a number of years, in parallel with the growth of computing 
capability and the increase of atmospheric data coverage. The several 
atmospheric and oceanic gcm's have now reached the point where 
reasonably accurate simulations of the global distribution of many im- 
portant climatic elements are possible and where their coupling into a 
single dynamical system is now feasible. This therefore seems to be a 
useful time to survey briefly these models' climate simulation capabilities. 

Here we have not attempted to present a detailed discussion of the 
various gcm's, as such descriptions are readily available both in the 
literature and in documents describing special models. Model reviews 
have recently been prepared by Robinson (1971), Willson (1973), 
Smagorinsky (1974), and Schneider and Dickinson (1974), and general 

196 



APPENDIX B 197 

discussions of the use of such models for weather prediction and for 
studies of the general circulation are available [see, for example, the 
review by Smagorinsky (1970) and also Haltiner (1971) and Lorenz 
(1967)]. A survey of the physical and mathematical structure of both 
regional and global atmospheric models is also in preparation for garp 
(1974). What has not been assembled heretofore is the comparative 
climatic performance of the various models, and this Appendix is an 
initial effort to fill this need for both the atmospheric and oceanic 
global gcm's. 

In general, any formulation that relates variables of the climatic 
system to the external or boundary conditions may be considered a 
climatic model. We can thus identify basically empirical and statistical 
climatic models, as well as' those that rest on the system's dynamical 
equations. 

Within the dynamical climate models, a wide variety of the type 
and degree of parameterization may be seen. At one extreme are the 
vertically and zonally averaged atmospheric models that address the 
mean heat balance at the earth's surface, such as those of Budyko 
(1969) and Sellers (1973). In such models, the transport of heat is 
parameterized in terms of mean zonal variables, which are in turn re- 
lated to the surface temperature. At the other extreme are the high- 
resolution global general circulation models or gcm's. In these models, 
the details of the transient cyclone-scale motions are resolved, along 
with the global distribution of the elements of the heat and hydrologic 
balances. Even these models, however, parameterize certain physical 
processes, in that they employ empirical or statistical representations 
of some of the subgrid scale processes in the surface boundary layer 
and in the free atmosphere and open ocean, such as the effects of 
diffusion and convection. 

Dynamical climate models also display a wide variety of parameter- 
ization with respect to time. This ranges from equilibrium or steady- 
state models, such as that of Saltzman and Vernekar (1971), to the 
gcm's that explicitly calculate the time dependence of the circulation 
in steps of a few minutes. With respect to their treatment of both space 
and time, therefore, a wide range of models exists, and each is suited 
to the investigation of particular aspects of the climatic problem. The 
gcm's (of both the atmosphere and ocean) provide the most detailed 
representation of the physical processes involved but require large 
amounts of computation. These models have therefore been used up 
to the present time to study only the climatic variations on time scales 
of the order of years (for the atmosphere) to centuries (for the oceans). 
The more highly parameterized models, on the other hand, provide 



198 UNDERSTANDING CLIMATIC CHANGE 

less detail but are capable of treating the longer-period climatic varia- 
tions with much less computation. Once they are adequately calibrated 
with respect to observations, an important use of the gcm's will be to 
generate detailed climatic statistics, from which parameterizations ap- 
propriate to the various statistical-dynamical models may be prepared. 
In the remainder of this Appendix we give our attention to the princi- 
pal atmospheric and oceanic general circulation models, for the pur- 
pose of indicating their present capability to simulate climate. Before 
presenting these results, however, it is useful to review briefly the 
historical development of numerical modeling in general. 

DEVELOPMENT AND USES OF NUMERICAL MODELING 

The basis for the mathematical modeling of the behavior of the at- 
mosphere was first unambiguously stated by V. Bjerknes in 1904. It is 
only in the last 20 years or so, however, that the means for carrying 
out such modeling on a practical basis have become available. These 
include adequate observations for model calibration and verification, 
a knowledge of the important physical processes and their parameteriza- 
tion, and the computers and numerical methods necessary to perform 
the calculations. 

The observational base for numerical modeling of the atmosphere 
has grown steadily since the 1940's and early 1950's, when the global 
radiosonde network began to take shape. The igy provided further ex- 
pansion, but the observational coverage still needs augmentation, espe- 
cially over the oceanic regions. The real breakthrough toward the 
global measurements necessary for numerical modeling has come from 
the remote-sensing capabilities of meteorological satellites; with the aid 
of suitable surface (ground-truth) observations, these are capable of 
providing the first truly worldwide observations of the air and ocean 
surface temperature, moisture and cloudiness, and elements of the heat 
and hydrologic balance. By using the numerical models diagnostically, 
there is then the prospect of deducing the accompanying global distribu- 
tions of other variables, such as the wind velocity. Such a scheme is 
the observational basis of the proposed First garp Global Experiment 
(fgge) in 1978. 

The physical and theoretical basis for numerical modeling has grown 
significantly with the development of the theory of baroclinic instability, 
the parameterization of moist convection, and advances in our knowledge 
of the behavior of the stratosphere and the planetary boundary layer. 
Our growing understanding of these processes has increased the pros- 
pects for improved weather forecasts. These hopes are bounded, how- 



APPENDIX B 



199 



ever, by the realization that the atmosphere possesses limited predict- 
ability, i.e., that there is a time range beyond which the local variations 
of weather appear as random fluctuations as far as their explicit pre- 
diction by numerical models is concerned. Present indications are that 
this limit lies at about two weeks' time. 

The key physical processes that control the longer-period variations 
of the atmosphere — those that are properly associated with climate — 
are largely unknown, although we are beginning to recognize the im- 
portance of a number of feedback relationships, such as the air-sea 
coupling and cloudiness-temperature feedback. Numerical models that 
incorporate such effects are our best tool to develop a quantitative 
understanding of their role, in climate and climatic variation. 

The computational base for numerical modeling has grown during 
the last 20 years in parallel with the development of successive genera- 
tions of high-speed computers, as shown in Figure B.l. This overview 
makes clear the interrelated development of numerical models, theory, 
and computer speed. Numerical weather prediction may be considered 
to have begun with the first successful numerical integration of the 





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and ocean. 



200 UNDERSTANDING CLIMATIC CHANGE 

vorticity equation (Charney et al., 1950), with the demonstration of 
the ability of baroclinic models to forecast cyclonic development 
(Charney and Phillips, 1953), or with the commencement of operational 
numerical weather prediction in 1955. Numerical general circulation 
studies may be considered to have begun with the simulation of the 
atmospheric energy cycle in an idealized model with sources and sinks 
of energy and momentum (Phillips, 1956), with the first successful 
hemispheric circulation experiments (Smagorinsky, 1963), or with the 
first extended global integration (Mintz, 1965). Numerical climate 
models for the atmosphere may be considered to have begun with the 
global simulation of the seasonal and interannual variation of the 
primary climatic elements (Mintz et al, 1972), although the modeling 
of climate by other methods has a much longer history. The numerical 
modeling of climatic variation, on the other hand, which addresses the 
coupled ocean-atmosphere climatic system, has only just begun (Bryan 
et al, 1974; Manabe et al, 1974a, 1974b) . 

The development during the past decade of numerical methods whose 
stability and accuracy can be suitably controlled has made it possible 
to carry out such calculations for extended periods of time. Even with 
today's fastest computers, however, the solution of the more detailed 
global numerical models proceeds only at a rate between one and two 
orders of magnitude faster than nature itself, and our ability to per- 
form the large number of numerical integrations required for the system- 
atic exploration of climate and climatic change requires the continued 
development and dedication of new computer resources. 

A similar pattern of development has occurred in the numerical 
modeling of the oceans, except that the rate of progress has been 
slower due principally to a lack of suitable oceanic observations. The 
data base for the oceans is fragmentary in comparison with that for 
the atmosphere, and there is no oceanic counterpart of the radiosonde 
or weather station network. The bathythermograph has been widely 
used to measure the thermal structure of the ocean's surface layer for 
the past few decades, but even this has not been done on a synoptic basis. 
The bulk of the data for oceanic temperature, salinity, and currents 
has been obtained in the course of occasional oceanographic expeditions 
or special observational programs. Even so, the number of direct 
velocity measurements is quite small, and our knowledge of the oceanic 
circulation is largely based on geostrophic estimates from conventional 
hydrographic observations. 

Our knowledge of the dynamics of the ocean circulation is also less 
complete than is that for the atmosphere. While the character of the 
vorticity balance of the ocean was first established by Sverdrup (1947) 



APPENDIX B 201 

and Stommel (1948), the role of the thermohaline circulation was 
demonstrated with a numerical model only a few years ago (Bryan and 
Cox, 1968), and the effects of bottom topography have been established 
even more recently (see, for example, Holland and Hirschman, 1972). 
Numerical models are proving of great value in the study of time- 
dependent behavior of the oceanic general circulation and in the analysis 
of oceanic mesoscale motions such as those now being revealed by the 
mode observations. The structure of these eddies and the role that they 
play in the oceanic heat balance is one of the principal unsolved prob- 
lems in physical oceanography. Other important questions concern the 
nature of vertical mixing in the ocean, especially in the surface layer, 
and the mechanics of the formation of deep and bottom water. Each of 
these can perhaps be most fruitfully studied with appropriate regional 
numerical models, in order to lay the foundation for their parameteriza- 
tion in three-dimensional models of the world ocean. But perhaps the 
most important problem of all from the viewpoint of climate is the 
interaction between the ocean and the atmosphere; the numerical 
modeling of this coupled system offers our best hope of achieving a 
quantitative understanding of the dynamics of climatic variation. 

Numerical models thus lie at the heart of the modern study of climate 
and climatic change: they complement (and may even be regarded 
as a part of) the observing system, they serve as tools for climatic 
analysis and diagnosis, and they offer the most rational way of assessing 
the course of future climatic events. Whether or not climate forecasting 
in the time-dependent sense ever becomes feasible, the use of numerical 
models to simulate the average or equilibrium climates of the past 
and the likely climatic consequences of various natural or anthropogenic 
effects in the future will justify their development. 



ATMOSPHERIC GENERAL CIRCULATION MODELS 

Formulation 

All general circulation models are based on the fundamental dynamical 
equations that govern the large-scale behavior of the atmosphere. This 
system consists of the equation of motion (expressing the conserva- 
tion of momentum), the thermodynamic energy equation (expressing 
the conservation of heat energy), the equations of mass and water 
vapor continuity, and the equation of state. When geometric height 
(z) is the vertical coordinate, these equations can be written in vector 
form as follows : 



202 UNDERSTANDING CLIMATIC CHANGE 

dV — ■*- dV — ■*■ 1 — 

-^ + V-VV + w-^- + 2nxV + — Vp = F, (1) 

g+P? = 0, (2) 

^ + V-pF + |^(pw)=0, (4) 

f + f.V, + .||=5, (5) 

P=p/«r. (6) 

Here K is the horizontal velocity, w is the vertical velocity, n is the 
rotation vector of the earth, p is the density, p is the pressure, g is the 
gravitational acceleration, 6 is potential temperature [which is related 
to the ordinary temperature T by the relation 6=T(p /p) K , where 
/?o=1000 mbar and * = 0.286 is the ratio of the specific heats], q is 
the water vapor mixing ratio, R is the gas constant for (moist) air, 
and V is the horizontal gradient operator. 

The terms F, Q, and S on the right-hand sides of Eqs. (1), (3), and 
(5) represent the sources and sinks of momentum, heat, and water 
vapor due to a variety of physical processes in the atmosphere and must 
be either prescribed or parameterized in terms of the primary dependent 
variables in order to close the system (l)-(6). The net frictional force 
F consists of the frictional drag at the earth's surface and the internal 
friction in the free atmosphere, as well as the changes of large-scale 
momentum due to smaller-scale processes. The net (diabatic) heating 
rate Q consists of the latent heat released during condensation, the 
heating due to the exchange of both long-wave and shortwave radia- 
tion, and the sensible heating of the atmosphere by turbulent heat fluxes 
from the underlying surface. The net moisture addition rate S consists 
of the difference between the evaporation rate (from both the surface 
and from cloud and precipitation) and the condensation rate. 

An important contribution to each of these source terms is the vertical 
flux of momentum, heat, and moisture, which accompanies cumulus- 
scale convection in the atmosphere. We may note that such convective- 
scale processes are not governed by the system (l)-(6) and must be 
represented in terms of the larger-scale variables. This parameterization 
is particularly critical for the net heating, because most of the latent 
heating in the atmosphere is accomplished by convective motions, which 
are also responsible for much of the cloudiness (see Figure 3.2). 



APPENDIX B 203 

The various atmospheric gcm's are each formulated in slightly dif- 
ferent ways and employ different treatments of the source terms. There 
is at present insufficient evidence to decide which particular formulation 
is the most satisfactory, and there is even more uncertainty regarding 
the most correct parameterization of the unresolved physical processes 
contained within F, Q, and S. A summary of some of the features of 
the better-known atmospheric general circulation models is given in 
Table B.l. Each of the models shown here uses generally similar pro- 
cedures to determine the ground-surface temperature (from an assumed 
heat balance over land and ice), the surface hydrology (with runoff 
permitted after saturation of the surface soil), and the occurrence of 
convection (from vertical .stability criteria depending on the moist 
static energy). Each of the models also incorporates the observed large- 
scale distributions of terrain height, surface albedo, and sea-surface 
temperature. 

Solution Methods 

All the atmospheric gcm's considered here employ finite-difference 
methods of second-order accuracy, with the dependent variables gen- 
erally determined on a spatially staggered grid with a resolution of 
several hundred kilometers (see Table B.l). Time differencing is also 
generally of second-order accuracy, with time steps between 5 and 
10 min used to maintain (linear) computational stability. Long-term 
(nonlinear) computational stability is inherent in some of the models' 
space differencing schemes, while others employ eddy diffusion processes 
to achieve this end. Various degrees of smoothing are also employed 
in the models' solution, in addition to that inherent in the finite-difference 
approximations themselves. Depending on the computer used, the 
number of model levels, and the frequency with which the radiative 
heating calculations are performed, global atmospheric gcm's gen- 
erally run between 10 and 100 times faster than real time. 

Selected Climatic Simulations 

In order to display the level of accuracy characteristic of present-day 
atmospheric gcm's in the simulation of climate, we have here assembled 
the results of model integrations drawn from recently published (and in 
some cases as yet unpublished) sources. To facilitate comparison, these 
are presented in a common format, along with the corresponding ob- 
served distributions. 



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APPENDIX B 205 



Sea-Level Pressure 



Although the various gcm's differ greatly in their resolution of the 
vertical structure of the atmosphere, each simulates the distribution of 
a number of climatic variables at the earth's surface. Of these, perhaps 
the distribution of sea-level pressure is the most familiar; it is shown 
here as simulated by four different models for the month of January. 
In Figure B.2 the average sea-level pressure simulated by the 11-level 
gfdl atmospheric model is shown for the months of December, January, 
and February (Manabe et ah, 1974b). Figures B.3, B.4, and B.5 show 
the corresponding average January sea-level pressure simulated by the 
six-level ncar model (Kasahara and Washington, 1971), by the two- 
level Rand model (Gates, 1972), and by the nine-level giss model 
(Somerville et al., 1974). In each case the observed average January 
sea-level pressure distribution is also shown. While the models' results 
differ in a number of details, these results generally show a useful level 
of accuracy. As might be anticipated, the largest errors (and the greatest 
differences among the models) occur in the middle and higher latitudes 
of the northern hemisphere where cyclonic activity is the most frequent. 
It should be recalled, however, that sea-level pressure alone is by no 
means a complete indicator of climate. 

Tropospheric Temperature and Pressure 

In Figure B.6 the average January 800-mbar temperature simulated by 
the two-level Rand model (Gates, 1972) is shown, along with the ob- 
served distribution. Although systematic errors may be noted over the 
continents, the simulated large-scale temperature distribution clearly 
reflects the positions of the major thermal perturbations in the lower 
troposphere. The average January 500-mbar height simulated by the 
nine-level giss model (Somerville et al, 1974) is shown in Figure B.7, 
along with the observed distribution. These results also clearly show 
that the mean position and intensity of the long waves in the westerlies 
are portrayed reasonably well in the simulation. 



Cloudiness and Precipitation 

Among the more difficult climatic elements to simulate accurately in a 
gcm are the cloudiness and precipitation. This is doubtless due to the 
fact that a substantial portion of the total cloudiness and precipitation 
observed occurs in connection with convective-scale motions, especially 




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UNDERSTANDING CLIMATIC CHANGE 



in the lower latitudes. As noted earlier, these processes must be parame- 
terized in the gcm's, and their accurate calibration is relatively difficult. 

In Figure B.8, the average January total cloudiness simulated by the 
six-level ncar model (Kasahara and Washington, 1971) is shown, 
along with a composite observed distribution for January and for De- 
cember, January, and February. With the exception of the equatorial 
region and the low latitudes of the northern hemisphere, the large-scale 
areas of maximum and minimum cloudiness are reasonably well 
simulated. 

In Figure B.9, the annual average precipitation simulated by the 
11-level gfdl model (Manabe et al., 1974b) is shown, along with the 
corresponding observed distribution. In addition to the large-scale 
precipitation pattern in middle latitudes, this simulation also portrays 
a number of the smaller-scale features, including the zone of heavy 
precipitation near the equator. Although this comparison is for a some- 
what longer time period than the others shown here, the difficulty of 
correctly parameterizing the precipitation process makes the skill of this 
simulation impressive. 

OCEANIC AND COUPLED ATMOSPHERE-OCEAN GENERAL 
CIRCULATION MODELS 

Estimates based on observed data show that the heat transported by 
ocean currents plays a major role in the global heat balance (Vonder 
Haar and Oort, 1973). A model that is to be useful for the study of 
climatic variation must therefore include the ocean as well as the 
atmosphere. As suggested by the simulations just reviewed, the speci- 
fication of a fixed ocean surface temperature in atmospheric gcm's is 
a strong boundary condition and may mask weaknesses in the models' 
simulation of the heat balance. The problem of climatic variation there- 
fore furnishes a major motivation for the accelerated development of 
numerical models of the oceanic general circulation. 

Relative to numerical models of the atmosphere, numerical modeling 
of the ocean is still in a primitive state. As previously noted, this is 
primarily due to the lack of sufficient data to perform a careful verifica- 
tion of the models and to parameterize properly the effects of the 
smaller-scale motions. The only large body of data presently available 
for verifying ocean circulation models is the collection of measurements 
of density structure. While these data were sufficient to calibrate the 
earlier analytic theories of the ocean thermocline, global numerical 
models require a much more extensive data base for adequate verifica- 
tion. 



APPENDIX B 219 

It is now recognized that many of the earlier studies, such as those 
by Bryan and Cox (1967) and Haney (1974) for idealized basins, 
as well as the higher-resolution simulations of Cox (1970) for the 
Indian Ocean and of Friedrich (1970) for the North Atlantic, represent 
transient rather than equilibrium solutions for the boundary conditions 
imposed. The extended integration of even more detailed numerical 
models will be necessary in the future, in order to design and calibrate 
adequately other simpler models. Such models will require less calcula- 
tion and thereby allow more freedom to carry out the large number 
of numerical experiments required. General reviews of numerical model- 
ing of the ocean circulation are given in the proceedings of a recent 
symposium (Ocean Affairs Board, 1974) and by Gilbert (1974). 

Formulation 

The principal dynamical components of an oceanic general circulation 
model are similar to those of its atmospheric counterpart, namely, the 
equations of motion, conservation equations for potential temperature 
and salinity, the continuity equation, and an equation of state. In addi- 
tion, an oceanic model should contain equations for the growth and 
movement of pack ice. 

In some problems of oceanic circulation, it is not necessary to treat 
the temperature and salinity separately, and these variables can be 
combined into a single density variable. In climatic studies, however, 
we are interested in the heat transported by ocean currents explicitly; 
and in many regions of the world ocean, particularly the polar seas, 
the density and temperature are not proportional. In these regions at 
least, it is therefore necessary to predict salinity as a separate inde- 
pendent variable. A changing salinity structure in the ocean may pro- 
vide the basis of climatic change mechanisms that have not yet received 
sufficient attention. 

In an ocean model, the equation of motion (1) may be simplified 
by treating the density P as a constant po (Boussinesq approximation), 
while the hydrostatic equation (2) remains unchanged. The thermo- 
dynamic energy equation (3) and the water vapor continuity equation 
(5) are represented in the ocean by conservation equations for po- 
tential temperature 6 and salinity s of the form 

-|^ (6,s) + V-V(B,s)+w^(6,s) = (Q,<r), (7) 

where Q and o- denote source functions. The continuity equation (4) 




220 




221 




223 



224 UNDERSTANDING CLIMATIC CHANGE 

may be simplified by considering the ocean to be incompressible, in 
which case we may write 

^ + V-F=0. (8) 

oz 

The oceanic equation of state may be written symbolically as 

P= P (0,s,p), (9) 

where the actual expression is a polynomial of high order, whose co- 
efficients have been determined by laboratory experiments. To close 
the system, expressions must be chosen for F [in the simplified form of 
Eq. ( 1 )] and for Q and o- in terms of the dependent variables. As in the 
case of atmospheric models, this closure is an important problem in 
the formulation of oceanic models and includes the parameterization 
of the mesoscale oceanic eddies. 

Solution Methods 

The predictive equations for momentum, temperature, and salinity 
given in the previous section are generally approximated by centered 
differences of second-order accuracy, with care taken to conserve both 
linear and quadratic quantities. The numerical methods that have been 
used successfully for large-scale models of the atmosphere are usually 
further modified by the exclusion of external gravity waves from the 
system. This permits the use of a time step 50 to 100 times larger than 
is possible for the atmosphere. This is accomplished by requiring the 
total, vertically integrated flow to be divergence-free, in which case 
it is possible to specify the total transport by a stream function. 

The numerical time integration of an oceanic gcm formulated in 
this manner proceeds by a combination marching and jury process, in- 
volving the explicit prediction of 0,s and V, and the iterative solution 
for the total transport stream function. Takano (1974) has recently 
introduced the implicit treatment of Rossby waves, which allows a con- 
siderably longer time step with little loss in accuracy for problems in 
which the emphasis is on low-frequency oceanic phenomena. 

Selected Climatic Simulations 

To illustrate the characteristic climatic performance of global oceanic 
gcm's, we here present comparative solutions from the recent models 
of Takano et al. (1974), Cox (1974), and Alexander (1974). A 
number of characteristics of these models are given in Table B.2. These 



APPENDIX B 225 

TABLE B.2 Characteristics of Recent Global Ocean Circulation Models 



Feature 


UCLA a 


GFDL " 


Rand ' 


Number of levels 


5 


9 


2 


Horizontal spacing 


A0 = 4° 


A0 = 2° 


A</> = 4° 




AX = 2.5° 


A\ = 2° 


AX = 5° 


Salinity 


No 


Yes 


No 


Depth 


4 km 


Actual 


300 m 


Horizontal mixing d 


Am=W 


^, = 2x10" 


/4if=7xl0° 


(cm 3 sec -1 ) 


A n = 2.5xl0 7 


A,,= W 


A„ = 5x\0 7 


Initial condition 


Isothermal 


Observed T,s 


Observed T 


Time span of experiment 


30 yr 


2.5 yr 


1.5 yr 


Upper boundary condi- 


Momentum flux, 


Momentum flux, 


Momentum flux, 


tion 


thermal forcing 


T,s specified 


heat flux 



"Takano et al. (1974); see also Mintz and Arakawa (1974) and Takano (1974). 

6 Cox (1974); see also Bryan et al. (1974). 

c Alexander (1974). 

d Here A M and A n denote the eddy coefficients for momentum and heat, respectively. 



models are currently undergoing further development, and similar 
oceanic models are under construction at ncar and at giss. It is a 
general characteristic of all such oceanic models that the circulation is 
dominated by the large values of viscosity, and further efforts are re- 
quired to extend the solutions into the less viscous and more nonlinear 
range. 



Surface Current 

The annual surface current simulated by the nine-level gfdl model 
(Cox, 1974) is shown in Figure B.10, along with the observed currents 
for February and March. The February surface currents simulated by 
the five-level ucla model (Takano et al, 1974) and the March 1 sur- 
face currents simulated by the two-level Rand model (Alexander, 
1974) are similarly shown in Figures B.ll and B.12. In each case the 
overall pattern of the large-scale circulation is simulated successfully, 
although in general the strength of the equatorial and major western 
boundary currents is underpredicted. We may note, however, that the 
ucla model's solution represents a 30-year integration, the gfdl solu- 
tion is for 2.5 years, and the Rand solution is for 1.5 years. Closer 
examination reveals that the simulated surface currents diverge from 
the equator somewhat more than do those observed, due to the models' 
effective averaging over the depth of the surface Ekman layer. 




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235 



236 UNDERSTANDING CLIMATIC CHANGE 

Sea-Surface Temperature 

The February sea-surface temperature simulated by the five-level ucla 
model (Takano et al, 1974) is shown in Figure B.13, along with the 
observed distribution. To some extent the agreement of the simulation 
with observation is due to the use of observed components in the sur- 
face heat balance condition. The prediction of low surface temperatures 
at the equator, however, is a feature entirely due to the model's internal 
dynamics. 

Coupled Ocean-Atmosphere Models 

As has been previously noted, a dynamical model adequate for the 
study of climatic variation should include the coupling of the ocean 
and atmosphere. The first attempt at such coupling was made by Manabe 
and Bryan (1969) for an idealized ocean basin and later extended by 
Wetherald and Manabe (1972). In such a joint model, the net fluxes of 
heat, moisture, and momentum at the air-sea interface are determined 
by the atmospheric model, while the ocean model in turn provides the 
sea-surface temperature as a lower boundary condition for the at- 
mosphere. 

These studies at gfdl have recently been extended to the entire world 
ocean, and the results of a coupled numerical integration are now 
available (Manabe et al, 1974a; Bryan et al, 1974). In this study, the 
nine-level gfdl atmospheric model was integrated for 0.85 of a year 
simulated time, while a twelve-layer ocean model was integrated for 
256 years' time. The annual sea-surface temperatures simulated in this 
joint model are shown in Figure B.14, along with the observed distribu- 
tion. The general level of accuracy may be considered satisfactory, 
especially in view of the absence of any specification of observed quanti- 
ties at the air-sea interface. Much further development and testing of 
such coupled models is required so that their potential for the study of 
global climatic variations may be realized. 

REFERENCES 

Alexander, R. C, 1974: Ocean circulation and temperature prediction model, The 

Rand Corporation, Santa Monica, Calif, (in preparation). 
Alexander, R. C, and R. L. Mobley, 1974: Updated global monthly mean ocean 

surface temperatures, R-1310-arpa, The Rand Corporation, Santa Monica, 

Calif, (in preparation) . 
Arakawa, A., and Y. Mintz, 1974: The ucla atmospheric general circulation 

model, Dept. of Meteorol., U. of Calif., Los Angeles, 403 pp. 



APPENDIX B 237 

Bryan, K., and M. D. Cox, 1967: A numerical investigation of the oceanic general 
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Bryan, K., and M. D. Cox, 1968: A nonlinear model of an ocean driven by wind 
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Bryan, K., S. Manabe, and R. C. Paconowski, 1974: Global ocean-atmosphere 
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Budyko, M. I., 1956: Heat Balance of the Earth's Surface, U.S. Weather Bureau, 
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Budyko, M. I., 1969: The effect of solar radiation variations on the climate of the 
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Charney, J. G., and N. A. Phillips, 1953: Numerical integration of the quasigeo- 
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Charney, J. G., R. Fjortoft, and J. von Neumann, 1950: Numerical integration of 
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Clapp, P. F., 1964: Global cloud cover for seasons using tiros nephanalysis, Mon. 
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Crutcher, H. L., and J. M. Meserve, 1970: Selected level heights, temperatures and 
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Environmental Technical Applications Center, U.S. Air Force, 1971: Northern 
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Friedrich, H. J., 1970: Preliminary results from a numerical multilayer model for 
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Kasahara, A., and W. M. Washington, 1971: General circulation experiments with 



238 UNDERSTANDING CLIMATIC CHANGE 

a six-layer ncar model, including orography, cloudiness and surface temperature 
calculation, /. Atmos Sci., 28:657-701. 

Lorenz, E. N., 1967: The Nature and Theory of the General Circulation of the 
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APPENDIX B 239 

tion to the equatorial currents of the eastern Pacific, Proc. Nat. Acad. Sci., U.S., 
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Sverdrup, H. U., M. W. Johnson, and R. H. Fleming, 1942: The Oceans, Prentice- 
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bourne, 53 pp. 



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